Misleading Pan Sizes?

I recently purchased several of the Good Cook brand baking pans from my local Kroger. They are all dark and non-stick, but I do not know the material other than metal of some sort. One reason I purchased these was because the dimensions are molded into the edge of each pan: 9”x5” or 8”x4” or 5.75”x3”. How convenient!

At least, those are the dimensions I thought I had purchased. I don’t normally carry my tape measure to the grocery store; so I just trusted those numbers were correct. It wasn’t until later when I realized the real dimensions from top inside edge to top inside edge:
the 9”x5” pans are really 9.5”x5.25”
the 8”x4” pans are really 8.5”x4.5”
the 5.75”x3” pans are really 5.75”x3.125”

Why the difference? I wonder how they got those numbers? At least the pans aren’t smaller than what I thought I was getting, but I’d rather have accurate measurements. What’s the point of molding these into the pan, only to get it wrong? More importantly, how will this affect my baking…Say if I’m baking a recipe that calls for using 8”x4” loaf pans but I’m using 8.5”x4.5” instead?   OR say if the recipe calls for two 9”x5” pans but I'm using three of my 8”x4” pans that are really 8.5”x4.5”?