A-type lenses and newer encode their aperture range in the connection pattern of (most of) the electrical contacts, also with them the relationship to aperture opening / lever movement was changed from what it was with K/M-lenses. The latter has been kept same since. Hence, the bodies can fully control the aperture from A-series forward: they keep the lens fully open for a bright viewfinder and stop down to what is set on the body for exposure (provided that the aperture ring is in the 'A' position, one of the contacts is dedicated for this). The body gets further information via the data pin, but it is not needed for aperture control.
edit:
see:
http://kmp.bdimitrov.de/technology/K-mount/Ka.html http://en.wikipedia.org/wiki/Pentax_K_mount