If manual PD measurement with a ruler is done at arms distance (~50cm) – how come it’s used as distance PD?

Whenever my PD is measured with a ruler it is taken at a distance of about 50cm.

But there is a big difference between infinity-PD and 50cm-PD. For example, an infinity-PD of 64 corresponds to a 50cm-PD of 60.7 (see conversion table between PD at different distances).

So how come distance PD is always measured manually at such short distance?