QM33120WDK1, STS_POA-STS1_POA is not same as PDOA

Hi!

I’m testing QM33120WDK1. In PDOA mode 3, I can read the phases of arrival of STS (STS_POA) and STS1 (STS1_POA) by accessing the sub-registers 0x0C000C and 0x0C0014. But the difference between these two phases is not exactly the same as the PDOA given by dwt_readpdoa(). They are always a few degrees different, and the difference is not fixed. Is there a specific reason for that?
Thanks!