6 bit colour is 2^6 x 2^6 x 2^6 = 262,144 colours
8 bit colour is 2^8 x 2^8 x 2^8 = 16,777,216 colours
Hence, a 6 bit monitor can only display 262,144 true colours (other colours are dithered) whilst an 8 bit monitor can display 16,777,216 true colours (or 16.8 million - Note, manufacturers incorrectly report this as 16.7 million).
However, manufacturers report the colour specs for a 6 bit monitor as 16.2 million to indicate that the monitor can display millions of dithered colours. I don't know why they chose 16.2 million, but the number is irrelevant - it could have been 15.7 million, 16 million, 16.014 million etc - as long as it is not 16.7 million (this is the true count for 8 bit monitors).