In DTM test ,nrf21540 out power is difference on Different frequencies

Hi,

During the testing of nrf21540DK conduction power, a strange phenomenon was encountered: in DTM testing, when the transmission power was set to 0dB, there was a significant difference in conduction power between channels 37 and 39. The conduction power of channel 37 is 4dB, and channel 39 is 8dB. Why is that?
The DTM example I am using is nRF5_SDK_17.1.0_de560 \ examples \ dtm \ direct_test_made \ pca10112

The test results are as follows:

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