Are the changed figures before or after the 60-second "cool down" on un-SKing/re-SKing?
The wisdom of Shadowe: Ghostraptor: The Shadowe is wise ...; FFM: Shadowe is no longer wise.
; Techbot_Alpha: Also, what Shadowe said. It seems he is still somewhat wise 
; Bull Throttle: Shadowe was unwise in this instance...; Rock_Powerfist: in this instance Shadowe is wise.; Techbot_Alpha: Shadowe is very wise *nods*; Zortel: *Quotable line about Shadowe being wise goes here.*
Also, did you fight the exakt same mobs and levels?
Cause certain villain groups give diffrent amounts of xp.
I've been looking at XP ratios myself. I'm don't know the exact details but I think I've got a rough idea how it works.
The XP you get when a mob goes down depends primarily on your combat level, the mob's level relative to this, the size of the team and whether the mob is a minion, lieutenant or boss.
The biggest factor is the last. You get 10x as much XP for a boss as for a minion. This may explain why you were getting 50 XP for some mobs and 500 XP for others.
Team members with different combat levels get different XP because the enemy con's differently to them. Note that the "combat level" of a side kick is one level below their tutor's. So in your case, the combat levels were 18, 20 and 21. I believe that this explains where the 3:4:5 ratio comes from.
The team size factor can be quite large. You will all get double XP if you're in an 8 man team! Obviously, this applies to all of you so it's not immediately obvious when comparing your combat logs.
I believe that the dev's intention is that the lowest level members of the team level fastest. This means that teams that stick together eventually end up at the same level.
I can't tell you whether the actual XP awards are correct or not 'cos I don't know what all the factors are. (Neither the wiki page nor Red Tomax's calculator match what I see in practice but they come out about right.)
I don't know about the cool down. That might explain why the numbers didn't change when you un-sk'd everyone.
I hope this helps a little.
Also bear in mind that the amount of damage you do to the mobs affects the xp award of they are damaged by multiple players.
Mind of Gaia lvl 50 Defiant's first Mind/Storm 'troller.
Deadly Doc 50 Dark/Dark Corr
and lots more on Pinnacle,Union and Defiant
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Also bear in mind that the amount of damage you do to the mobs affects the xp award of they are damaged by multiple players.
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I believe that only applies if the mob was damaged by someone who wasn't a team mate. If your team takes it down you get full XP. (Maybe that's what you meant.)
Thanks for the replies, and this:
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I believe that the dev's intention is that the lowest level members of the team level fastest.
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sort of explains it all, together with the sk cool down which i didn't know about either. 
It's a 'warm up' as well. If you enter a mission un-sk'd and you then get your sk, it seems to take an age before you start getting the right level of exp. It's maddening sometimes when the fights are going on and you're getting nothing at all for them. This is another change made which I don't understand at all.
So im on a three man team, the other two are mates.
I have a 18 tank sidekicked to a 21 controller.
The third is an 18 tank not sidekicked.
On missions, the cnt was getting 300xp per "arrest", i was getting 400, and the non sk tank was getting 500.
So me and the other tank swapped sk.
Cnt was getting 50, i was getting 30, and the now sk'd tank was getting 40. Same mobs.
So finally we tried with neither tank sk'd, and i was getting significantly less than the other tank.
*Also all of us were still only getting 30-50 xp.*
And the Cnt was still getting the least.
Now i have never analysed this before, but being on TS makes it easy to compare, and now i am sure i dont understand what i thought was a simple equation.
Anyone shed any light on this, and help my poor old brain out?
edit- to clarify that the only way any of us got decent xp was if i was sk'd. The other tank being sk'd did not help.