Computers "think" in binary...
The Apple II had a 3-bit colour mode (hi-res) with 8 colours (two of which were duplicates), so it's far from non-sensical. 3-bit colour on an SE sounds like a clever hack to make the most of scarce resources.
FWIW, Apple is currently being sued over their 6x6x6-bit iMac display (262,144 colours) which they tried to pass of as "millions" of colours, so as you can see there are no hard rules concerning powers of 2.