How could you make the measurement result no changing when multiple measurement windows enabled

When customer used RFmx SpecAn to measure one CW signal(SG setting power as -10dBm) , they found below problems:
1.When he only enabled the "Channel Power" measurement, he see the "Total Carrier Power" was -10.13dBm;
2.When he enabled the "Channel Power" and "Spectrum" at the same time , the result of "Total Carrier Power" is only -9.68dBm.
Why is there the big difference for different measurement windows enabled status ? 

1.When we only enabled “Channel Power” measurement , we can check the LO parameters as below picture :

2.When i enabled the "Channel Power" and "Spectrum" at the same time , we re-check the LO setting parameters as below :

We can see there is a difference on the parameters LO Offset and Downconverter Center Frequency；

3.When we set the "Downconverter Frequency" still same value manually when i enable one measurement "Channel Power" and two measurement "Channel Power" and "Spectrum". We can see the measurement result of Total Carrier Power was still no changing; 

设置LO参数不变，我们可以保证CHP的测试结果不会因为“Spectrum”测试项打开或者关闭而变化；