this is only because charismatic is written as "25% less xp to upgrade" (I don't care about the exact number) (and as a side note, it is badly written) instead of a "you get 25%more xp on combat" promotion. +(free xp is boosted by 25%; otherwise it would be borked.)
/me quote himself

hahaha.
well what I meant by "badly written" is that it says "-25%xp needed for upgrade" while it feels as a "I have only an advantage of 15% in xp".
Indeed, one
understands that a unit with charismatic would upgrade to a level when it has accumulated 25%less xp than without charismatic : thus the unit will level up at 1,75xp (2), 3,75 (4 instead of 5), 7.5 (8 instead of 10), 12.75 (13, instead of 17), 19.5(20instead of 26), 27.75(28, instead of 37) ...Etc
whereas, If I remember correctly, the charismatic trait work in fact like this : you need 25% less (
rounded up)
to upgrade to next level; and the rounding up accumulates : thus each further level require the unit to gain/spend ; +2xp, +2.25(3), +3.75 (+4), +5.25(6), +6.75 (7), +8.25(9), +9.75 (10). Thus the unit change levels at : 2xp, 5xp, 9, 15(instead of 17), 22(instead of 26), 31 (instead of 37).
And thus, while it is written "-25%xp needed for upgrade", it feels only like a "-15% xp needed". (15 is 12% of 17, 22 is 85% of 26, 31 is 83% of 37).
That could have been corrected error by using
rounding to closest instead of
rounding up. Rounding to closest would change the xp needed to upgrades to next level to : +2 (1.5), +2(2.25),+4(3.75), +5(5.25), +7(6.75), +8(8.25), +10 (9.75)... This would lead to level changes at : 2, 4, 8, 13, 20, 28, 38. those are the same values for change of level as if I each level (not xpneeded for next upgrade) needed 25%less xp.
such minor correction in the "rounding" could have given back the feeling of "-25% xp needed".
(as RifE's xp has two decimals, I think this rounding up effect disappeared)