Carrier offset in USRP2+2.4/5GHz daugherboard


I tried to obtain the carrier offset by using the PLL block in GRC.
The transmitter is the Zigbee radio at 2.4GHz. Then, the signal is
downconverted in USRP2 souce. Two are connected by SMA cable.

As shown in the result, the frequency offset can be as large as 200KHz.
it correct?
From the standard and datasheet, it seems like 10-20ppm tolerance for
and 40ppm for Zigbee device.

But I am still not sure why such large offset appears.

I will appreciate for any suggestions! Many thanks