I will explain why with the combined perforation it is possible that not all the holes in the corners are aligned.
I found the explanation in the Michel catalog (also valid for French stamps which, as far as I know, were only punched combined)
In the corners at the top the holes are aligned, at the bottom they are not.

And yet it is a combined perforation made vertically.
I took the explanation from Michel:
die durch leicht verschobenen Kammschlag Linienzähnung vortäuschen =
which simulate line perforation due to the slightly shifted crest.
and if you are not convinced, you can check if the holes on the sides are on the same horizontal (which is less likely with perforation in line)
It is possible that the combined perforation is made horizontally on the sheet and then the holes are aligned on one side and you can continue the verification methodology.
I'm sorry that in the previous post I didn't give these explanations, which led to the conclusion that I was wrong.