I wouldn't be surprised if "1-inch" battlemaps bought in America were actually 25mm squares instead of 1 inch. The difference would be minimal and the vast majority of customers wouldn't even notice that they're using a metric map with a switched label. The width of the gridlines is wider than the difference between 25 mm and 1 inch. You'd need to measure 6-7 squares before there would be a 1/8 of an inch difference.
I wouldn't be at all surprised either; nor would I be surprised if the "25mm" battlemaps are actually 1 inch square grids produced in the US, but labeled for international sale. I have no preference as to whether you conceive of and call the diameter of a US 25 cent piece 25mm or 1 inch. I'm just curious as to whether 25mm/1inch grids are available in countries where 1 inch isn't a common unit of measurement.