We had something similar a good few years back.
We went in to buy <X> square metres of tiles, selected the tiles, put in the order and got <X-1> square metres.
When we checked this we got some sort of story along the lines of "Ah sure, Mister, I only know square feet, so when I get an order in square metres I convert it and then key it in, so it's not exact, but sure like it's nearly right."
The ordering system would then take his square feet figure and convert it back to square metres, but using a different conversion factor - leading back to a different number of square metres than requested. The fact that the tiles were shipped, priced and stocked in square metres didn't seem to put this guy off converting everything and getting it wrong.
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