There are times when I feel harder than usual. There have been moments when my dick was rock-hard and feeling like it was about to explode, and on those times my dick was definitely a little bigger. There have been times when I've gotten a solid erection, where I can get off, and where I could probably fuck a girl, but it was still slightly bendable, and I think it was a little smaller than normal.

Erections aren't exact on ANY guy. Erections have to do with blood flow; in any flow, not much is perfectly exact, unless you stop it completely. If that's the case, well, I think you have to go see a doctor. Not too sure on that one.

In any event, the amount of difference can vary. Some guys get essentially the exact same erection every time, and some get varying degrees of hardness depending on certain factors.