Just to clarify what has already been stated. The meter will give you an idea of the exposure before you take the shot. The histogram will tell you the exposure after the fact. You can then make adjustments in the next shot. One possibility for the discrepancy is whether you are using matrix, spot or center weighted metering.
To Eric's point about camera settings and the histogram. Is this a subtlety or is this measurable in stops?
BTW- I just tried to post this a few minutes ago which did not seem to register. Apologies if I posted 2x with slightly different wording.