There are multiple regulatory requirements that affect the max EIRP transmit power. The generic in-band power limits are just one of them. Out-of-band and band-edge emissions are examples of other regulatory requirements that are affected by transmit power. These will vary from radio to radio, which is why you will see different power limits for different APs.
Also, the hardware capabilities are different for different APs. In the case where we're allowed to transmit at the max capabilities of the harwdare, that will also result in different limits for different APs.