Ok, I tested it, and the two designs do have the correct (and same) range, so fuel consumption must be working properly on a macro level at least.
Edit: Is there a chance that something regarding fuel is calculated on the 5-day increment to "correct" the fuel usage? If he only advanced one day then it wouldn't have shown.
Edit:
Wait, wait, stupid question (but sometimes the stupid things are the ones you don't see), you weren't flying them in the same task group were you?