Most quilting rulers are marked in inches as most quilt patterns are in inches. The one centimetre difference could be a rounding effect when converting the inches to centimetres.
However, there do tend to be some quilting rulers that differ in size by half an inch , so you might find both 6x12" and 6½ x 12½" rulers, for example. Half an inch rounds down to 1cm so this might also account for the difference.