You're correct. There really is no way to account for every scenario. I've made this point before elsewhere as well, maybe not the same way exactly.
For instance I feed a DIY food out club makes that is very "dense" and one commercial food maker told me he thought it would "overload your filtration".
The guideline is meant to get you in the general range of where you need to be. There are a few "guideline" factors that fall into this category.
Also, just to play devils advocate, if I feed 2 cubes/day on a consistent basis and I don't have any measurable N or P over a long term period (years) and the scrubber is my only filtration, literally the only filtration, and I harvest > 60g/wk of algae, and each feeding is about 6g (2 cubes or so)...pretty much exactly what you're saying is shenanigans is actually happening.
I have 2 tanks in this exact situation. I have another smaller tank like this also, which I feed proportionately the same.