Let me add a little to this, LYNX. It's not just the timeframe, but also whether or not you look on a weekend, middle or beginning of the week, or holiday. Also, snapshots, Friday TOD's, scenario's...all change the numbers, fairly quickly. Add to this, If a particularly favorite map comes up in an arena, and word gets out(Tank town maps have this effect, in particular) An exodus starts from one arena to another, with the unbalancing consequences. Back in the days of the Single MA, The number shifts were a lot less noticable, because there were fewer factors to drain from the 'pool', and the one MA was more balanced through all of the time zones.
I've read posts by Tilt and a few others that have been more adversely affected by lower arena populations when on Euro-time. I can see there point. Hopefully, a satisfactory solution can be found for the Euro-players...But the arena's don't usually fill to capacity until Prime-time in the U.S.(About 5-11P.M. EST.)