This was my understanding too i.e. that the stepped piston diameter
could put pressure on the seal
if there was fuel behind it on the return stroke of the piston. This could
potentially cause the seal to fail. The words in italic are meant to mean that there is an element of chance/probability/luck involved in all this. Some folks have been fine up until now and it looks like you have been lucky so far, Micky. I guess I was not so lucky.
Again, perhaps JKMs pictures are better than words to explain the effect: