Before any wood is cut for a set like this, the whole thing is drawn once - full size, in pencil, on a flat board. Not a scaled drawing on paper. Full size, one to one.
The reason is the relationship between the chair and the stool. The stool has to put your ankles at the height your knees are already at, or your legs hang or fold and the recline stops working. That height depends on the seat height, which depends on the rocker arc, which depends on the recline of the back. Change any one and the others move. On paper at a tenth scale, a two-millimetre pencil line is two centimetres of real error, and by the time three of those have stacked up the stool is simply the wrong height.
So the board carries the true curve of the rocker, the true angle of the back, the true seat line and the stool sitting where it belongs against them. Every component is then taken directly off that board with a stick, marked and cut - no arithmetic in between, and so no rounding.
It is an old drawing-office habit and it is why the two pieces look like they were designed together. They were, on one board, before either existed.