This is a guess ...
It may be that this approach to aperture priority is a reflection of the fact that many cameras don't permit you to set intermediate shutter speeds, but do permit you to set intermediate apertures.
So if you ask the meter to give you a useful reading near as possible to f/5.6, it may be recommending:
1) a shutter speed easily set on most cameras, plus
2) the necessary less-than-one stop adjustment to your preferred aperture.
Hope this helps.