Why Do These Perf Holes Look Oval?

From time to time I see this kind of perforation on Palestine Mandate stamps, that rather than there being round horizontal perforation holes (like the strip on the left), the ones produced look more ovalish (like the strip on the right). Does anyone know why that might be?

The perforation measure on the left strip is somewhere between 13.25 and 13.5, and on the right strip its about 13.5.

I have seen instances of different sized holes along the same axis and for this I've been told it's because the perforation pin has been replaced, and not always by a pin of the same gauge. But here the hole sizes (on the right strip) appear to look consistent.

Are the stamps on the left on thin paper (earlier) and are the stamps on the right on thick paper (later)? Maybe the larger perforation holes help to separate thicker paper more easily.

I forgot to mention the paper in my original post: both are thick vertically ribbed (only that one is yellow-green and the other a darker green color)

Were the stamps comb perforated?

I had to look it up :-) yes, all were comb perforated in this series...

Regarding replaced pins: do you if a replaced pin which is visible in a certain position on a perforation (eg horiontal) would appear in that same position on all the horizontal perforations or just on a specific line?

It depends on the number of rows/columns that the comb has and how many times the comb gets repeated so that it most likely would get repeated on just some specific lines like for example once in every four rows.