IIRC, the non-p variants, when 250Kbps mode is requested, silently falls back to 1Mbps mode. If you already are using this library, you can test by doing the following, which will return 'true' or 'false'.
radio.isPVariant() ;
Or you can determine what the hardware is currently configured for via:
radio.getDataRate() ;
which will return one of:
RF24_250KBPS, RF24_1MBPS, or RF24_2MBPS
The fallback logic occurs because 250Kbps is '10' bits. The non-p variants ignore the high bit (of the two bits, '1' of '10'), resulting in a single low bit '0' being latched to hardware. This is the same as requesting RF24_1MPBS. And given its the longest range, universal data rate, it seemingly makes for a good default.
Also as a minor nit, a 256Kbps mode is not available on the hardware. I'm guilty of this too. So to be clear, its a 250Kbps mode and NOT a 256Kbps mode. The distinction is important to remember because of the defined enums. Otherwise, you'll constantly mistype one of the enums.