I though the same thing. And we don't carry balances from one month to the other (and a credit rating of just under 800 for it). Then my DH (that stupid MBA in accounting & finance

), showed me daily online one month that, yup, that balance had interest incurring daily (and adding the previous days balance into that)
with no carried over balance. I'm a hard learner. And, OK, he has a degree dealing with numbers and still had a hard time figuring it all out.
We then went through every cc we owned (a whole whopping 3, one of them business) and read the fine print--all did that. Everytime we get an offer in with great bonuses, we think about switching--compare interest rates, the bonus points, and
how interest is figured. Sometimes, usually, how the interest is calculated makes a lower rate card a higher rate in the end. They do have to make money; otherwise there is no point in loaning you the money.
We still use our bonus card, but twice a month where we can pay it off within 24 hours or only when we have to (booking travel) between those dates.
And, heck, not even all rebates are equal. I want a Disney cc, just for the picture

, but the bonus isn't as good as our rebate (cash back) card.