I'm sorry to disagree but, for example, blown in fiberglass/cellulose insulation needs to be about 12-14" thick to have a rating of R-30. Hence, if your joists are 10", then you'll end up compressing the mix, thus, dramatically reducing the R-factor.
If you're referring to R-30 rolled fiberglass insulation, then you're right. In this case, it's as 'thick' as the joists and you'd be correct. I guess all you need to realize is that insulation comes in a LOT of forms, including spray foam.
I'll assume you either don't understand or fully understand what 'R-factor' means. Once you do, you'll certainly understand what COMPRESSION does to the R-factor. My best to ya and hope this helps.