Blissey has base HP of 255, and HP is calculated differently than other stats.
With 252/31 spread, a Lv. 100 Blisey has like 712HP.
Now, what if a fully Dynamaxed Blissey could hit more than 999HP? Do you think that would break their spaghetti code?
Earlier games had problems when numbers exceeded values they expected (and there are at least two examples you can do legit with some preparations)