their lengths are identical, there is no overlapping video, and all the options in the generated media window are set exactly the same, yet my second credit roll is a good second shorter than the first one.
I agree with Hunter - don't forget that blank lines count as lines in this context - so if your "shorter" credit roll has more empty stuff at the end that the other roll, that's your culprit.
The length of the event on the timelines may (appear to) be identical, but is the time identical in the duration box at the top right of the credit roll input window?
I would guess that the second credit roll has more Header lines than the first, which causes more pages to have to fade. Since the duration is the same, it must fade the pages faster in the second one to get them all in.
So if your first credit role is 7 sec with two Header lines, and the second one has 4 Header lines, set the second duration to 14 secs and they should now fade at the same rate. In other words, make sure the duration to Header ratio is the same in both credit rolls.