Was the O2 you calibrated with the same as you were using on the dive?
If not, check the O2% of your onboard O2 - I know that mine is only about 98% at the moment due to lack of discipline in flushing whips etc. If yours is actually down around 95% it will give the results you are seeing.
Otherwise, is there a depth difference between your pressure guage and your sensors (how long are your arms
)? Or were you fizzing like mad at the time?
Mike