It probably depends upon the time and place in medieval Europe. I know that usury (either the charging of interest or the charging of interest greater than a certain rather low (by modern standards) rate--I'm not sure which since I haven't made this a topic of my study) was forbidden by the Western Church. Consequently, non-Christians--mostly Jews as they were the primary non-Christian minority in Europe at the time) filled that economic niche.
If usury was lending at any interest, then the rates quoted would probably be typical rates of interest from Jewish moneylenders. If usury was lending at more than a minimal amount of interest, I would imagine that the rates previously quoted were what was approved by the church for Christians and that Jewish moneylenders may have had different interest rates. (And may well have been more willing to lend out money).
Assuming that both figures are accurate, it seems most likely to me that the low figures were those permitted to Christians by the church and the higher figures you found were those charged by Jewish merchants who were actually permitted to be in the moneylending business. In any case, it seems to me that those rates are artificially low and probably are a result of church prohibitions against usury. In locations where there were no such prohibitions (such as pre-Christian Iceland, for instance), the rates might have been higher--and those or the Jewish interest rates might be a more realistic source for a D&D campaign where usury is not forbidden to the majority of the populace.
It would also be interesting to find out how joint ventures were treated in medieval days--for instance a prospective inkeeper in an outlying town offering to go into business with a wealthy merchant in the city--the merchant pays for the majority of the materials, in return for a certain percentage of the inn's income or profit (or perhaps, simply for a set amount of money per year--in the medieval era, things that were supposed to be percentages like the various tithes were generally set at a fixed amount and left there for years; it's conceivable that such could happen in business as well). In an environment where usury is not permitted to most merchants, that type of arrangement seems like a potentially attractive alternative to loaning out money at 1-2%.
Numion said:
I thought that interest rates were much higher in medieval times. If rates are that low, doesn't it also mean that rents are very low too?
EDIT: At least a little googling came up with much higher results for medieval interest rates: 12-30%.