A footstool is easy to get wrong by an inch. Too high and the knee lifts; too low and the heel carries the leg. The number that matters is not the chair's seat height on the drawing - it is the seat height once someone is sitting in it, after the cushion has compressed.
So we measure it loaded. The chair is finished and upholstered first, then a weight is set on the seat and the compressed height is taken from the floor. Only then is the stool's frame cut. On a deep leather seat like this one, the difference between the empty and the loaded measurement runs to well over an inch, and a stool built to the empty figure sits noticeably tall.
The stool's own top is crowned slightly, so we take that into account too and measure the stool loaded as well. The pair is checked together on the workshop floor before either goes to finishing.
It is a small piece of arithmetic that nobody sees and everybody feels. When you sit down and put your feet up without adjusting, that is where it came from.