When I was in the old 56th we saw this all the time because we flew in groups with the same load outs and used WEP on takeoff for the first 7,000ft. We experienced the same phenomenon, usually with the same players pulling ahead in FSO because we flew in formation as part of the immersion factor.
Not all players connections are equal in this game. Anyone who has ever tried flying formations knows that.
This drives me nuts in events too. I always advise whoever is the lead to never ever be at full throttle. Just be fast enough so that you're not walking away from anybody. And when the other guys ask what his throttle settings are the reponse should be--whatever you need to be to keep formation with me! In RL if all the planes were of the same type and config then of course you could all set the same mani and rpms and have it fairly easy in keeping formation. But here on the interwebs ya just can't do it that way. Somebody's position is gonna get updated a little faster or slower depending on their connection relative everybody else's and that's just how it is.
On a related note I have found that on taking off in the game for my first hop I have the same problem. If I just go full throttle and fire up the engine I'll start roling but it's just not at actual full throttle accorking the gauges. If I cycle the throttle and rpm it comes right up to max and isn't a problem the rest of the night. Cal is right. No explanation. I just deal with it