In general, a "natural straight edge" stamp is philatelically considered a "fault", even when the edge is natural, the philosophy being, for majority of collectors, it's just not desirable.
You can see in your stamp as well that the perforation is into the bottom of the design. So the overall size of the stamp remains reasonably the same. (Sometimes, you may get a "Jumbo" if it's near an edge, with big margins around the stamp). So it's not a surprise to me that you see a small portion of the top stamp in the horizontal block line.
Note these were also made into booklet pains, so straight edges are not uncommon.
Here's a few I just plucked out of a stock book.