Simple explanation: your XP bar mod is broken.
Once upon a time, many XP bar mods took advantage of a bug in the way Sony provided XP info -- that info wasn't rounded -- to create "XX.X%" tenths-of-a-percent displays. Last summer, Sony fixed the info, so that all numbers were rounded, which caused those displays to no longer be accurate. But many XP bar mods have never been corrected, as many modders still seem to think Sony "broke" the info, and are waiting for a "fix" that will in fact never come.
(As I've said in the past, it would be very nice if Sony would provide the info in the old manner, or in both the old and new manners, but it's become obvious that that's not going to happen, so we really need to deal with what we've got.)
As to exactly what's happening, suffice it to say that whever the decimal value of your XP display is 5 or more, the whole number value is one higher than it should be, as it's been rounded, since Sony intends that number to be seen only as a whole number, and not as part of a tenths-of-percent display. When you see 94.3%, for example, you really do have 94.3% XP, but when the display jumps to 95.6%, it's really just 94.6%. And when you see 100.9%, you're actually at 99.9%.
-- Darryl (Milquetowst)