I don't understand the experience systems

Hi, on my server i tape command "/xp pseudo xx" and i learn xp but i don't understand the calcul.

if i'm level 0 and give me 14xp on one command, i'm now level 2.
if i'm level 0 and give me first 7xp, i'm level 1 and after 7 more xp i'm not level 2, but finally i have the same number of xp oO

 

This should answer your questions:
http://www.minecraftwiki.net/wiki/Experience

The system is a bit more complex then you thought.

 

lol, i see this page, it's wrong !

if i'm level 1, E = 2*7=14 experience, ok, i tape commande
/xp me 14 (i'm level 1 when i taping this)
now i'm level 2 AND ~30% of xp bar...

i have made many test and result are
level 0 to go level 1 need 7xp
level 1 to go level 2 need 10xp
level 2 to go level 3 need 14xp
level 3 to go level 4 need 17xp
level 4 to go level 5 need 21xp

with latest recommended build craftbukkit
It's crazy ! ! !

 

Apparently this is a known bug, extract from the wiki:

Normally when gathering XP:
From level 0, if you get 7 xp you go to level 1. From level 1 to go to level 2, you need 14 xp additionally, so you'd really need 21 xp total to get to level 2.
According to this bug, the /XP command will fail, and will think you only need 7 xp for going to level 1 and another 7 xp for level 2, this is why when using the command you will reach level 2 with only 14 xp. The bug will think that all levels require the amount required to go to the next level. So if you are at level 0, and give 35 xp you will reach level 5, even though you should really only reach level 2.
So this is apparently a vanilla bug, and not Bukkit bug.