As a workaround, if you add the C = A * B formula to the raw data (before creating a PivotTable), then it will sum C exactly as you expect.

Alternatively, if you use a SUMPRODUCT formula outside of the PivotTable, it too will work as you expect.

=SUMPRODUCT((A column =A)*(B colmn = B), Column being summed)

Note that when building the formula, you should type a cell address for A and B rather than clicking on the cells. Otherwise, Excel will use a GetPivotData function reference, which may not be what you want.