JB73, Hitech specifically mentioned that the "Bish default country" is a myth. The players are said to be allocated randomly at first log on.
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In my observation, it is something about the tendency of players in a certain time zone. In European time zones the numbers are usually equal, but once it shifts to the US prime time, things begin to change.
The problem is, the numbers drops down for a certain country - typically Rooks - when other countries start gaining them.
Since the numbers stats are usually even at European prime time, this could only mean that the players newly logging on at US prime time are choosing certain countries exclusively. For whatever reason it is, US players log on to mostly Bish and Knits, and they are not logging on to the Rooks. So, for 5~6 hours till the prime shifts from US east coast to the West coast, the Rooks are in a constant banging.
The things start waning as finally the prime leaves US West coast, crosses the pacific and lands at Asian timezone.
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Maybe there's some tendency of US based players that avoids the Rooks as their country of choice?