It seems that I missed a couple of the posts that were later edited, but can infer that the gist was that someone noticed a subtle difference even between seemingly identical bottles over the years. This would be very possible in my opinion, and actually likely. Even though there has been a gradual change in the industry towards synthetic ingredients there are surely still quite a few natural sources for much of the scents, and even more use of said natural components from those scents with historical lineage.
I'll give you an unrelated example, orange juice. Having grown up in south Florida, orange juice was a common drink. Grandma had seven orange trees in the backyard and there were at least three other immediate neighbors that had at least one large tree in theirs. (that we had permitted access to) While there were some trees that produced better oranges than others, some of those were small trees that also produced much more limited quantities of oranges, so blending of the juice was the key. The variations of the "blends" would vary from year to year and even during the season. It was sometimes a challenge to combine the correct ratio of sources throughout the year and still maintain a sweet, but drinkable juice that kept you coming back for more. You could taste a sampling of oranges from one tree and think that it was what you expected, but when blended it would not taste correct and require further doctoring with others to produce the results sought.
This would surely translate over to the various fragrance sources and while they might all still smell correct in their original form, when blended with the remaining ingredients could easily generate "batch variations" that could result in noticeable differences. Things like interactions with one's body chemistry, or other interactions like one's choice of deodorant, aftershave, shaving soap, hair products, or even just plain old longevity. Longevity, now there is something that one usually only thinks of in overall terms, but rarely is there but one component to the fragrances that we enjoy. No, there is a blend, and if one component lasts a little bit longer than usual, and a different component has a shorter lifespan this batch, then the transition from top, to heart, or heart to base notes will be altered. Combine this with different years and their doubtless variations of temperature, humidity, environmental smells, etc. and you will surely end up with a frag that will perform uniquely for that particular combination, and person.
A fragrance company that often gets hit hard by the critics is Creed, this company touts itself as using (at least a high percentage of) natural ingredients. It really doesn't matter how much a batch smell alike in the lab, under controlled conditions, in the real world there are additional variables that come into play, and these will influence the behavior of the fragrance to some extent. With a high profile, and expensive fragrance like the Creed's this gets talked about and folks take heed. But with something more commonplace like the Guerlain that is under discussion in this thread, it easily passes under the radar of the more common user. With that one it will take the passage of several years to introduce the differences and often only the direct comparison of bottle to bottle (purchased over time, and therefore from a different "batch") to actually notice that a change has happened at all. It will be more likely to be noticed when there has been an obvious change like a new bottle style. That's when a higher percentage of folks will think to themselves "I hope it is only the bottle that has changed" and actually compare, head to head.
This board (B&B) is made of an extremely small percentage of the population, and this particular forum of this board is smaller yet. It is likely only folks like us that would actually notice the subtle variations from batch to batch, and year to year. Average folks simply use the stuff and enjoy it. Replacing bottles when they run dry, and sometimes even only after they have been out of that one for a bit. Sure, they may have a small selection to choose from, but will usually only make those choices based on plans for the day. Everyday, for work they may choose something like clockwork, when going out for a nice dinner it may be something else, and another for a nightclub. But these choices will likely be the same every time, predictable, and reliable. If it continues to smell like they think it should, they will be happy, and they will be likely to think it smells the same because it comes from the proper container, not from some detailed analysis of the progression of the notes from said fragrance (or fragrance product.) It is more likely still, that when they do doubt weather the fragrance in question is actually the same, that they will ask a loved one, or common acquaintance to smell them and check. Undoubtedly the response will be that they indeed do smell the same. After all, they still smell like they always do, be it something like Old Spice, Guerlain Vetiver, Aqua Velva, etc. The individual still smells like the one doing the smelling remembers, and this is the ultimate vindication that the scent has indeed stayed the same. (Even though we know better.)
I ride a motor scooter most of the time. I get between 80-115 miles to the gallon when I do. On those rare days that I drive my old beater of a car, I get 8 miles to the gallon, YMMV, mine sure does.