EA = East Asia
In most cases they use different builds of the same firmware but the rest is the same.
Exception are the DIR firmwares NA from 1.20 (1.30 DIR-655) which run with a newer version of the sdk and sharepoint.
Here is also 3G function disabled (invisible).
Another minor difference: most WW firmware has Dual WAN option for Russia. (not 2 WAN ports but two different connection types at the same time)
NA DIR firmware has advanced DNS function
As far as I know there no newer device anymore which changes frequencies with the firmware.