@frikkr In this case it wasn’t that.
I don’t know how I could have been so stupid…
The thing is, I used a UV Sphere with 6 segments and 12 rings. Because of the 6 segments it can look round or oval, depending on the viewing angle. If you look at it from the top you can clearly see, that, in this case, it is smaller along the X than along the Y-axis:

Because it has twice as many rings, those ‘angles need less rounding’ than the angles from the ‘hexagon’ when subdivided. Hence the ‘squeezing’.
It’s just a topology thing: it consists completely of stretched rectangles; that’s asking for trouble;)