No I haven't confused them. Its 40x30 so that the RGB lines will be visible.
I also noticed that If set to 10x10 the number of RGB pixels is not 10x10 its way more. So I can conclude that if you set the mask counts to you game resolution its no where near pixel perfect.
Lots of things need to be fixed with the shadow_mask_alpha option and everything thats related to its settings.