There's a ton of little stuff like that. It kind or ranges from frustrating (having your bullets pass right through enemies due to how hitscans are handled) to potentially exploitative (
enemies receiving splash damage multiple times). All in the name of getting this sucker to run on a 386, I imagine!
As far as why the hitscan thing happens, it's due to moreeeee optimizations. Doom has a data structure called the blockmap. In the WAD file, this will carve the map into 128x128 unit slices, logging all of the linedefs that fall within each of those slices. After the engine loads the level, it also adds things to the list, tracking enemies, items, players, and decorations in the blockmap structure as well. This is used to accelerate collision, but it does have one notable downside. Here's a visual of how one scenario may look:
The numbered squares are individual blockmap blocks. The green object is the player, and the red object is a large enemy (let's say, an arachnotron). You can say with quite a bit of certainty that the player is in block 3, but what about the arachnotron? Well, when it comes to thing-to-thing collision everything will work correctly, since the projectile or object is physically travelling, but what about hitscans?
One of the shortcuts that Doom does is to use blockmaps to determine whether it's even possible for a tracer to hit a target. The problem here is that it only factors in the origin. As a result of this, despite the fact that a large amount of the arachnotron spills over into blocks 1, 3, and 4, only tracers that pass through block 2 are considered. If you were to aim at this arachnotron's left or right legs with a chaingun and pull the trigger, you'd miss every single shot. While the problem gets far worse with wider enemies, this can even be observed with pinkies and even former humans (if you're particularly unlucky).
It's important to note that this affects anything that uses hitscan tracers, including the BFG9000. The BFG can deal over 3500 damage to a single target if the ball and all of its tracers land within said target, and getting this high of a damage roll isn't all that uncommon (roughly 1 in 8, which is pretty favorable in terms of RNG). This is more than enough to kill the spider mastermind with one shot (she only has 3000 HP, and the odds of your BFG rolling that much damage is roughly 50%!), but in practice it's a bit more rare due to this issue. Because she's a whopping 256 units wide (large enough to fit in four blocks, minimum) it requires you to aim at just the right part to ensure that as many tracers as possible land within the block that her origin falls in. Given the size of her hitbox, I'm not even sure that it's possible for all of the tracers to land (you're going to be standing a minimum of 128 units away from her origin point, after all) so the odds of getting that coveted one-hit kill is pretty low in practice, with 2-3 being a more realistic number.
(G)ZDoom doesn't use the blockmap to determine hitscan collision, which means that every single unit within the mastermind's 256x256 hit box can be affected by a tracer. If you get close enough, your odds of getting a OHK on her skyrocket right up to that theoretical 50% figure. It's impossible to not kill a spiderdemon in two BFG hits unless you simply can't get close enough.
Even before I got into this level of engine analysis I wondered why bosses went down so much faster in ZDoom. I'm glad I know now, at least!
