do a search on caps and you'll find enough reading to kill ya. Basically when HTC made 2 LW arenas out of one (to keep the population under 450) one arena would fill to 450 people, and the other would be 50. The 50 complained they wanted to play with the other 450, but seeing as HTC doesn't want the caps over 450 he tied the arenas together. As the population rises closer to the moving cap in one arena, the cap is increased in the OTHER arena, This way the arenas stay more equally populated.
It may not be perfect, but its the way it is, learn to live with it.