As far as I can tell under D&D 5E rules, you ALWAYS round down any time you get a fraction. Even if the fraction is 0.9, you round it down.
Please quote that rule here. And if true, then, it means D&D doesn't understand math.
There's one more general rule you need to know at the outset. Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.
- Page 7 (Introduction) of PH 5e.
It has absolutely nothing to do with (not) understanding math. Perfectly normal for e.g. a software developer.
