I would expect exactly this.
For any easel I'd expect the nomimal size to be given for the paper-size to be fixed. With a glass-plate easel the useful size would be identical to the paper size, for an easel with blases I'd expect the blades to reduce the useful size.
This concept of designation is especially usefull with easels in mind where one can switch between blades and glass-plate.
But I see your point.