Hotels, yes. But there is a sizeable number of long term snowbird tourists who escape to Thailand for 3-4 winter months, they will rent an apartment usually. Counting them together with the two week hotel tourists and dividing total number of days spent by both groups by number of farang visitors will produce an average of 4 weeks per farang.
Besides, if i stay 2 weeks in one hotel then move to stay 2 weeks in another hotel/city, do they count me as an average 2 weeks farang from hotel records?