Why are tires that fit 622mm wheels called 700C
As stated in this answer or this link, the idea was originally that there were 700A,700B,700C tires+wheels which all had an outer diameter of 700 mm with A tires being the thinnest and C tires being the thickest (so A had the largest rim and C had the smallest rim). Eventually, the C variant won out, and people varied the outer diameter by mounting different tire sizes on a 700C rim.