NRF24l01 Channel switching
You're getting confused with number bases. A number prefixed with 0x is hexadecimal. That's base 16 - not base 10.
0x125 is actually 293. If you try squeezing 293 into an 8-bit register it will truncate to 37.
Channel 125 is 0x7D in hexadecimal.
I can't recall off hand how big the register is for the channel number, but either 7 or 8 bits would suffice (probably only 7 bits actually used) so the highest channel number you could possibly enter would be 127, which is 0x7F, after which it will start at channel 0 again for 128 or 0x80.