We are a French gallery also proposed its own version of the size of stretchers. From the same table. And not really explain why such dimensions. Simply saying that they enjoyed in France, and all frames are made under these canvases.
This is apparently connected with the traditions and the traditions of an unusually strong in Europe than it is famous. World wide and everywhere the standards are different, as well as the voltage of electricity, not all countries are equal.
While yes, uncomfortable, stretchers had to be done under the order.
Paul, I do not think that 1cm significantly affect the picture.
The way, there is the size of Feng Shui.