|If you want to check the Month and the Year you have to evaluate each criteria individually.|
SUMPRODUCT uses a TRUE-FALSE array (evaluated as 1 or 0) for each criteria to determine which values to SUM.
If you want to see it in action, select the cell with the formula and click on Tools...Formula Auditing...Evaluate formula. Clicking the Evaluate button will allow you to "single step" through the formula so you can see the arrays as they are evaluated.
Where each criteria is TRUE; you get a 1, where it's FALSE you get a 0. Once all of the criteria arrays have been evaluated, the magic begins.
If all criteria for a given value are TRUE, you'll essentially have:
1*1*ValueX = ValueX.
1*1*ValueZ = ValueZ
If one or more of the criteria are FALSE, you'll have something like:
1*0*ValueY = 0.
After all the PRODUCTs have been evaluated, the results are SUMed, thus the name SUMPRODUCT.
ValueX + 0 + ValueZ = The sum of the values where all criteria are TRUE.
The new formula would look something like this (all on one line)