I also believe that if your partied with others of different level, the high level ones get a LITTLE more exp (so you all won't get the exact same number).
But, it's not that much higher. I think it's high enough so that you all get approximately the same percentage for a mob relative to your level.
Of course that does not remain proportional once someone is too high level for the mobs.. but I'm not sure of the exact figures.
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