I note on the PentaxForums that Royce Howland, fellow Calgary photographer has discovered a possible glitch in the Pentax, in that he can't reliably use the f stop ring on some lenses when in manual exposure setting. I had not discovered this because I always put the lens into A mode where the camera sets the f stop, after all the camera comes with front and back dials specifically for the purpose, but still, if you can do it, it should work.
When I tested my camera, the same problem happens - for some odd reason, when in manual mode and setting the f-stop on the lens, the camera thinks it is two stops wider, and that gap stays as you change f-stop. This only seems to happen with FA lenses, not the A series manual focus lenses, and not the new lenses that of course don't have f-stop rings anyway.
For me, it's a non issue, just something for people to be aware of - use the body to set f-stop, lens aperture set to A.The proper aperture is set and recorded.