DAX - Total does not add up

Hi guys, 

I need a bit of help as I can't figure out how to write a DAX function to retrieve a correct total for the "Total Products Sold Last Month" column.

Currently my dax for the "Total Products Sold Last Month" is as follows:

It does exactly what I am trying to achieve, look at the product release date, go a month back and retrieve a number of sales I.

However, when looking at the total of "Products Sold Last Month" it does not retrieve the correct total.

Could you please assist.