Okay, I just deleted a post because, in retrospect, it had incorrect information. It was, indeed, just that I was getting progressively better at the small section I was playing.
From my new test, I'm calculating that Veteran difficulty creatures have 70 percent more health than Story Mode creatures. I didn't check Stalker difficulty. Resources appear to be the same, at least at the very beginning of the game. Not sure about damage taken because I gave myself unlimited health so I could relax and not miss any shots and not feel I needed to shoot at invisible guy before he became visible.
So, yeah, there's a significant difference.