It has nothing to do with the server. The cap system is a best compromise to help balance the arenas. If you have a better suggestion on how to balance the arenas, then feel free to send us an email about it.
The twin Late War arenas operate under a dynamic cap. The single arenas for Early and Mid War do not need a dynamic cap as there is only one of each. If there were two of each of those, which there was for a while, then they would also be under a dynamic cap.
This topic has been discussed ad nauseum on the board and include detailed explanations from both HiTech and Pyro on this matter.
I am closing this simply due to the sheer volume of discussions about this topic are available via the search mechasnism. If you cannot find an answer there, then there is a good chance you will never find an answer. It also appears some very good information has been provided, which answers the questions, but possibly not the answers some are looking for.
If you want an explanation from HiTech or Pyro, then drop me an email.