It's not just to do with the RGB depth, because how far that reaches into the available gamut depends on the backlight type and purity, amongst other things. All 24 bit tells you is that it can attenuate the white backlight through red, green and blue filters with 8 bit resolution each.

This page seems to have a good summary:

http://compreviews.about.com/od/monitors/a/LCDColorGamut.htm

70%-ish NTSC is normal for most emissive displays, and some LCDs where power isn't an issue. Portable electronics have tended to lag because you can save a lot of battery by limiting the backlight power.