For divvying up experience after a kill there are many modifiers.
Basically, add up all the levels in the group (a level 23, level 25 and level 30 make up a group, so total is 78) and divide your level by that total (if you are the level 23 then this would be 23/78 = ~ 23.487) giving you the base %, so in this case you get 23.487% of the total exp for the kill. BUT, that is before any modifiers such as class(I'm not totally sure there are still class modifiers any more, but there used to be, so I'll include it here), race, double exp AA, double exp potions, zone you are killing in, difference in YOUR level compared to the mob you were killing (mobs less than 5 levels below your level qualify for exp bonus). Each of the modifiers is applied separately to each group member after the base % is established, so the modifiers that add exp are not taking that exp from someone else.
Given all this, 2 people soloing the same mob will get a different amount of exp from that kill. Say the mob is level 25 two times in a row when it pops. The first time a level 25 comes along and kills it solo. Next pop a level 31 comes along and kills it. The 25 toon gets the bonus for the mob being less than 5 levels below his level, while the level 31 toon doesn't get this bonus. Both kills are in the same zone, so they both get the same modifier there. If the 25 is a halfling he gets a bonus. If the 31 is an iksar he gets a negative bonus.
There are other modifiers too, such as a chanter using a charmed pet versus a chanter using shiny bob (chanter summmoned pet). The charmed pet shares the exp awarded at the end and shiney bob never gets exp (no, you can't ding your charmed pet). The amount of exp the chanter shares with the charmed pet is determined by whether the chanter did any damage to the mob during the fight. If the chanter let the charmed pet do 100% of the damage for the kill than the charmed pet takes a bigger portion of the exp. As long as the chanter does at least 1 point of damage the charmed pet is reduced to his minimum share of the exp (about 33% I think).
There are probably a few other situational times exp gets modified, but I think this covers most of it.