Movie accounting voodoo, what is the logic?

I think you are looking at this a bit incorrectly –

First of all, you are oversimplifying the movie-making and financing process. It's not as monolithic as you make it seem. Sometimes a major studio agrees to pick up you project, you shoot in their sets, you use their equipment, personnel, etc, they distribute your film. Cool. Usually, those studios, wanting to only make money, are going to be risk adverse. If you are a huge name, with a track record of making big money, with a blockbuster concept, they'll go all in for you.

They are also going to micromanage and make you compromise the artistic vision for mass appeal. (Hypothetical examples, not claiming they are real) – "We think Alan El-helani is a very gifted actor. But this is a 200M film, so we want Christian Bale to be the Prince of Egypt." "And that thing where the firstborn children all get killed? If we give them all a painful rash, instead, we're more likely to get Oprah to endorse it, plus then the moms who are going to watch will identify with having to deal with kids' rashes." Or "yeah, Jack Reacher is supposed to be a six-foot five-inch bear of a man, but let's cast Tom Cruise." "Instead of the bad guy escaping and killing the hero's best friend, let's have the hero blow him up with a hand-held rocket launcher, with a really cool one-liner before he shoots it." Etc.

So, what if you don't have a big-name, blockbuster, mass-appeal McMovie to make? Then, more likely, you, the movie maker, will use their sets, resources, distribution network, etc., but you have to assemble your own special effects people, do your own location shooting, and instead of them financing your deal, you pay them for use of their resources, pay for the other resources you need, and you have to find investors to back your movie.

So, if I line up investors, 20, 30 or more and scrape together the 100M, what happens when I only make $75M? Do they just make it up "next time?" Having lost 25% of their investment, what makes you think they will give me money for my next project? Having lost their money, what makes you think they will have more, period? For some investors, that would be it. Money in, no money out, money gone.

Even if it was part of a larger enterprise, you are saying that a movie losing money isn't a bomb because a studio would have other, non-bombs. That doesn't change what the movie is. If a successful movie doubles or triples the investment, then, yes, a movie that loses money would be considered a "bomb" by comparison. If the potential is there to make a lot of money, and it doesn't, that is a failure.

Let's say I run a toy-making company, and we make tons of profits. One of my product development people decide to put out a line of three-toed sloth action figures. We invest resources, and, when it goes to market, sales are really poor, and we fall way short of breaking even. Does the fact that I also had a different line of bikini zombie video games that made a lot of money somehow make the action sloths not a failure? As a successful company, who gets promoted? As a successful company, is the guy who pitched and spearheaded that project going to wind up cleaning toilets or collecting unemployment? Possibly.

They call them bombs because they fail to meet the minimum expectation or threshold to not be considered a "bomb."

Having said that, movie studios are infamous for accounting voodoo, especially when they have any kind of arrangement where someone else gets paid based on a cut of revenue or profits.

It's a weird thing. Many movies that don't recoup on release will eventually recoup though post box office licensing. (Netflix, Amazon, Xfinity, etc.)

The higher the budget, the higher the risk and the lower the ROI.

The greatest ROIs in film history all come from low and micro-budget films (i.e. Blair Witch, Paranormal Activity, Little Miss Sunshine, My Big Fat Greek Wedding, Crouching Tiger Hidden Dragon, etc.)

There is a lot of voodoo. Many smaller studios are made up of a series of shell companies. I assume at the level of a Sony Pictures, the accounting gets even more complicated.