Then Edward-san posted the obvious question - "are you sure it's not a bug that should be reported?"
I was convinced that the shots had always been fired from a very low position but on checking the original game, they do not start as low as they do in ZDoom. It's hard to estimate but in the original game the shots seem to come from somewhere in the area indicated in this pic:

In ZDoom they start very low and often catch on low steps and so on, exploding long before they reach the player. This does not seem to be the case in the original but the very low res and lack of a "freeze" command in the original meant it was hard to get an accurate idea of exactly where they came from. I'm pretty sure that it is higher up than ZDoom anyway.