90 degrees would be up, but because in computers the Y axis has traditionally incremented downwards, C2 follows suit and that means 90 degrees is down.
There are a few different ways of doing it - we could have made 0 degrees point up - but 0 degrees points right due to the way the math works out. Using:
x += speed * cos(a)
y += speed * sin(a)
then 0 degrees ends up being right (because cos(0) = 1 and sin(0) = 0). If we changed this it could cause problems, especially with third party plugins that might assume the above formula works when C2 does it differently.