Been using a Kickr paired with Zwift recently due to an injury. I also have the stages and have noticed the exact same issue. A little research points to Stages in some case (not predictable) being noticeably low. Two issues. Stages does left side only so there could be a large discrepancy in leg bias? Second it appears most people report the low reading correcting with the new stages version. Not sure which you are running but I am running the original version meter with updated firmware. All told I am considering switching power meters in the near future.