Not a duplicate of http://scifi.stackexchange.com/questions/107675/why-does-mark-use-hexadecimal-to-communicate

ASCII can encode a question mark. Its code in hexadecimal is `3F`. Since a question mark only appears once per question and most questions would be fairly long (i.e. many characters) why would Mark Watney waste precision (17 positions vs 16) to save on one code (`?`, length 1, infrequently used vs `3F`, length 2)? Also, it would seem that controlling the rotation in 16ths would be much easier than 17ths as 17 is a prime number whereas 16 positions can be constructed by repeatedly halving the perimeter of a circle. Was this simply dumbing it down for the audience?