Why do medieval death-toll figures for the Mongol conquest of the Khwarazmian Empire differ so drastically from what modern historians consider plausible?

Medieval chroniclers gave city-by-city death tolls historians regard as impossible given known population sizes: Ibn al-Athir claimed 700,000 killed at Merv while Juvayni recorded 1,300,000; Juvayni separately reported 1,747,000 for Nishapur and a claim of 1.2 million for Gurganj that is treated as almost certainly an exaggeration. Modern historians hold that affected cities could only have supported a fraction of these reported populations.