the K10D, from my experience, has a problem with respect to metering manual apature lenses.
I documented this in the following post,
https://www.pentaxforums.com/forums/241716-post69.html
the same behavior occurrs when you take an automatic apature lens out of the A position.
my suspicion is that pentax uses the apature information from the lens to "calibrate out" most of the exposure error, but when you take the lens out of A mode, this function is disabled. I demonstrated this once with my 70-200 F2.8 sigma lens also.
if you are over exposing in manual, I suspect you are shooting at apatures of F5.6 and smaller