Double Density vs. 99 min. CD's

Based on my limited understanding of them, it seems like Double Density CD’s offer the extra space by reducing the tracks closer together… but when it’s done on Double Density discs, they can’t be read by normal CD drives, whereas 99min CD’s can be, and they seem to achieve the extra space the same way…
Why is this?

99min CDs have a track pitch which is below standard (1,5-1,7 µm). However, it’s still large enough for quite a lot drives to read it.

I’m not sure if DD-CDs are real CDs (especially as to pit length).