1 = 21
5 = 23
10 = 26
Obviously not exact but near enough to give some idea as to experience boost.
You're confusing percentages with actual XP. 5 INt is getting you 2 more XP points or 9.5% more XP. 10 INT is getting you 5 more XP points or ~24% more XP. Still lower than I would like, but a far cry from 2.5% and 5%.