Its you ECL. (Effective Character Level)
Being a sorc with only 1 level of pally your bro will be level 6 BUT have an effective level of about 3-4. He will get exp based on his effective level. This is to make sorc/wizards etc. better able to cope with the game as they are weaker earlier.
You have 4 fighter levels and 1 bard. The bard does have an ECL decrease until you hit level 10 but this isn't really noticable with the fighter levels. So you are level 5 and your ECL is also level 5. You will get less exp per kill than your brother. If you took more bard levels so you were something like 4 bard/4 fighter you'd see a difference in early exp for kills.
All classses become even when they hit level 11 though. You will still end up about 1 level behind him though.
For SERIOUS ECL cheese try sorc 5/cleric 5/RDD 10. You'll hit level 20 somewhere in the middle of chapter 3! <img src="/ubbthreads/images/graemlins/eek.gif" alt="" />
