While doing research I stumbled onto this term, and it seems to be more of a video issue than camera.
F-Stop = Focal length Stop (Theoretical value)
T-Stop =Tested stop (tested value}
In what I was reading the difference the two is small and not worth testing for still cameras, but it is for video cameras. Since I assume, perhaps wrongly, that you use your still camera body option to shoot videos, are you using this "T-Stop"?
Wow! Haven’t seen a T-stop lens since I was in the Air Force! We had them for our mopic cameras and that was WAY BACK in the late 60’s!