What's the actual freq. when input freq. is wrong

GNU Radio
1

I use two USRP N210 (daughter boards are XCVR2450 supports freq range in
2.4GHz - 2.5GHz) to experiment with tunnel.py in
gr-digital/example/narrowband. I followed the instructions of README and
varied the parameters.
When I set freq in 2.4GHz to 2.5GHz:
./tunnel.py -f 2.42G -c 50
It didn’t work, both the transmitter and the receiver only showed TX
without RX and it didn’t response when I ping.

But when I set the freq to 300M (which is not supported by daughter
boards)
./tunnel.py -f 300M -c 50
It worked and both the transmitter and receiver showed TX and RX and
response when I ping. And it also gave the following warning:
UHD Warning:

  • The hardware does not support the requested RX frequency:*
  • Target frequency: 300.0000000 MHz*
  • Actual frequency: 2400.0000000 MHz*

UHD Warning:

  • The hardware does not support the requested TX frequency:*
  • Target frequency: 300.0000000 MHz*
  • Actual frequency: 4500.0000000 MHz*

Clearly the 4500MHz is not the real TX frequency. What is that?