Here's a couple of threads here on eq2interface.com about it:
XP percentages rounding incorrectly
A (Theoretically) Simple Exp Bar Fix
LU 24 and XP question
A couple of threads over at the official EQ2 forums too:
Experience Percentages Displaying Incorrectly
is the EXP tooltip rounded up?
Experience Percentages Displaying Incorrectly
Still after all this, I've yet to get some sort of developer at least acknowledgement of the bug. This bug has been around since LU 24. 2 LU's later, no advance on this. Keep trying to kick up dust about it, no luck so far.