Sumif() help

Gurus,

I'm stuck with this formula. I have a spreadsheet with two tabs. On one is a list of Standard Window Types in different houses and the quantity of that type of window for that house. The Window type can be repeated many times with different quantities.

On my second tab I have a list of all the unique windows listed only once. I want to know the frequency that a window is used in all these projects. I'm using =SUMIF(Quantity!$A$2:$A$1567,Usage!A2,Quantity!$B$2:$B$1567). The funny thing is I've got a total of 3,213 windows in these projects but the sum if returns a sum of 3302 (obviously something is wrong.

To add to my confusion when I sort the usage results with the most used window on top they don't sort in the right way.

I've attached a file showing the problem. The sum of the qty columns is at the bottom.

Thanks for you help.