That's what this whole discussion is about. Apple has created background art that measures 5120x2880, which is presumably the resolution of the next generation iMacs and Apple Thunderbolt Displays. How do you drive an ATD at that resolution with a single cable? If you're paying attention, that is not twice the pixels of the current 2560x1440 panels, but 4x.
Nope. 1 Gbit = 1,000,000,000 bits. There are no base 2 antics involved here. 20 Gbit = 20,000,000,000 bits.
And when you're calculating the bandwidth requirements of a display, you need to account for various types of overhead. More "pixels" are required than are actually displayed. For VESA CVT (Coordinated Video Timings), there are blanking intervals, front porch, sync, and back porch in both the horizontal and vertical dimensions. 5120x2880 at 60 Hz with reduced blanking ends up being 5280x2962. So 5280 * 2962 * 24 * 60 = 22,520,678,400 bit/s, which is pretty darn close to 22.52 Gbit/s. Thunderbolt 2 provides exactly 20 Gbit/s per link to the upper layers, and DisplayPort 1.2 with HBR2 offers 17.28 Gbit/s over a 4-lane main link.