Hi Kurt,
your basic maths seems sound... but, as Neil mentions, there can be other factors involved.
Are you doing a TIN to TIN (or Object to Object or Object to Model) or a TIN to Plane calc?
If you are doing one of the first types, try doing a TIN to Plane (or Object to Plane) and see what the area result is.
Typically, when doing an Object to Object (or similar), the area will be the smallest overlapping area of the two data objects.
If your data isn't too large, maybe you can ZIP it and post it here and I could take a look. Maybe provide a sanity check..