Xp lvl goes back in time

This bug is on smp
What I expected to happen was...:
I enchanted a axe with my 30 levels, then i logged off. When i came back i had 30 lvls again! Later i tried to get to 30, i got to 17 and had to log off. When i came back i had 14!

Steps to Reproduce:
1. get some xp
2 write down how much you have
3. enchant somthing
3. log off
4 log back on and compare