Trying to create a difference column in a matrix table

Hi,

I have a matrix table which shows the sales for products this year and last year. All I want to do is add a column which shows the difference between these two columns. I've been trying all afternoon but getting nowhere.

Here's the source data (much simplified version of my real data, but the same dilema exisits!)

And my matrix table looks like this:

All I want to do is add a third column of values to the matrix table to show the difference between TY and LY. For example Product A would be 5, Product B would be 0, Product C would be 10. But when I do this it doesn't display as I'd expect, it shows a strange difference for each year:

Does anybody know what I'm doing wrong?