Standard water color paper versus stndard frame sizes?

I have a bit of a quandary. Why are the standard sizes of watercolor blocks of paper never the same as the standard sizes of frames? It always seems to be an inch off. I can find 10X14 or 11X15 watercolor blocks, but I can only find 10X13 or 11X14 frames. Is there a conspiracy? If so, I don't know what it's trying to accomplish. Or is it because all the companies seem to be foreign and they have different standard sizes in that particular country. (They always annoyingly seem to write 'watercolor' with the British spelling, 'watercolour'.) However, that explanation doesn't make sense either since isn't most of Europe using the metric system? So why would they have the block dimensions set to an even inch?

If anyone out there has the reason, I'd be very interested in finding out.

Standard paper sizes in the UK still go back to the old imperial system. So the sizes are based on the A0 size (which probably was related to the size of a printing press bed? not sure). These they go down by halving the size: So A1 is half of A0, then A2, A3, A4 (standard letter size here), etc. They are given in metric now but I can't imagine that anyone ever though 21cm x 29cm (A4) were a nice convenient numbers to remember. *grin* So it doesn't explain the sizes you see, but maybe somewhere along the line they just started rounding up or down to the nearest inch?

Or it's a conspiracy. Much more likely.

Speaking of the relationship to frame sizes, watercolors are USUALLY matted so that the size of the frame is based on the dimensions of the matt, not the paper itself. Some framers like very wide matt borders while others are more economical. In other words, you'd buy a frame large enough to accomodate the watercolor paper PLUS the matt, with the outside dimensions of the matt matching the standard frame.

On the other hand, you'll find that canvas stretcher sizes more regularly match the frame size since paintings on canvas are not matted.