About 12 months ago a begin shaving with a straight razor and I'm about 1 and half months new to saving with a DE. I bought a 1953 Gillette Super Speed 40's style to learn with and a 1963 Gillette Adjustable to experiment with.
I don't want come off as over thinking this, but wanted to share an aha moment. Most experienced DE users may already know this.
When I started to shave with the fixed Gillette Super Speed, It shaved close but the next day I could shave again if I wanted to. When I shaved with my straight razor the beard did not seem to come back so quickly. I thought and thought why the difference. They both seem to shave well. Then it struck me after looking at the Gillette Adjustable. It was the safety rail. On the Gillette Adjustable More blade exposed made for a closer shave. When I set the Gillette Adjustable to 9 I got a shave more like my straight. With that said not exactly. I rarely nick myself with my straight razor and did few times with a setting of 9 on the Gillette. For me a 5 seems to work well with no nicks.
So my conclusion was that the position of the safety rail on the DE is what determines how close the hair is cut and a Straight razor is all about technique when it comes to a close shave since it has no safety rail.
Additionally, the position of the safety rail on the DE is what people refer to as level of aggressiveness (how close it will shave/expose blade).
An there you have it.