According to Ehrman (Misquoting Jesus, p. 84), John Mill (1707) examined readings in 100 Greek mss, and concluded there were 30,000 variants. Ehrman notes that we now have 57 times (his figures) the number of witnesses available to Mill (p. 88) and that there are between 200,000 and 400,000 variants (p. 89), which, incidentally, is more variants than there are words in the NT (p. 90).
Why should there be approximately 57 times more witnesses and only (very) approximately 10 times more variants? Were Mill's figures bloated? Or do subsequently discovered mss reveal ever lower proportions of previously unknown variants? What role do different estimates of the versions play in this?