I don’t quite understand your workflow. Correct me, if I am wrong:
You have 2 units, one as a rover and another one as a base, right? You obtained RINEX from both units and corrected it in RTKlib (rover correction against base RINEX). But, during the measurement, you had also RTK, that was supplied by your base. So, in your CSV file you have points, that are corrected via RTK, but in your pos file, you have PPK. And the averaged solution of PPK point extractor is different from what you see in CSV file (averaged by ReachView). Am I correct?