rollover math wrong?

So I keep seeing these deposit bonuses with rollover requirements, right? The numbers always look off to me when I try to track how much I've wagered. Like, if the promo is "10x rollover," I’ll deposit $100 and get a $100 bonus. By my math, 10x of $100 is a thousand bucks, but some places act like you need to bet $2000 before the rollover is cleared. Other times, it seems like only my bonus money is counted, or sometimes it's just my cash. Feels intentionally murky.

Does it depend on the games you play or are they all just counting on us not double-checking? Anyone else feel like the site math never adds up quite how you expect? How do you guys keep track of what counts towards rollover?