With you bringing up KoF, I'd also like to add that judging games solely by raw sales is going to hurt the ratings of not just unorthodox fan-driven works, but also quarter-munching arcade games like Space Invaders and free-to-play games like, dare I say it, Fortni-(Burns to ash upon invoking the name of the Cursed Game)
But speaking seriously, while "Sell this game and see how many copies are shipped" is one of the most prominent strategies, it's not the only strategy, and judging games by sales alone is a rather shaky strategy, to say the least. Not to mention games getting warmer reception later on and the extra variable of ports muddying the waters further.
What I'm saying is, while sales can be used as a bit of an indicator for how big a series is, they're just a fraction of a greater whole, and sometimes circumstances (Like piracy, as you mentioned) means that sales are a smaller fraction than normal. Short version being, Sales aren't everything.