Question about perforation

I wonder - how is it possible to have perforation similar to 11:11:11:11 1/2 (like Germany BIZO Mi#99IYP - see below) and not to have perforation 11:11 1/2: 11:11? I guess for each stamp 99IYP we should have neighbor in the sheet (to the left from 99IYP) with perforation 11:11 1/2:11:11.

Michel does give a explanation. They say the following.
Sheets that were perforate with 11 sometimes had no perforation on the left(pfenning values) or right(mark values). These were later perforated and for probably technical reasons this could only be done in 11.5.

So the stamps are on the outer edge and do not have a neighbor with a mirrored perforation.(i believe even to be found at the bottomside of the sheet)

The stamp to the left or right is 11 but I have not found information on these pairs yet.

Please note that this particular series is a minefield concerning perforations. my reply is most certainly not complete, but does give you a goood idea of what happened

Kind regards, Johan.

Johan - thanks! Now I found a small note in Michel. Definitely it is very detailed, but enough for now.