While researching on Shtut, WIKI says that the reason for that forgiveness is either הלכה למשה מסיני or human psychology.
Indeed, Rambam Mechira 12,3 writes:
"הָיְתָה הַהוֹנָיָה פָּחוֹת מִזֶּה בְּכָל שֶׁהוּא. כְּגוֹן שֶׁמָּכַר שְׁוֵה שִׁשִּׁים דִּינָר בַּחֲמִשִּׁים וּפְרוּטָה. אֵינוֹ חַיָּב לְהַחְזִיר כְּלוּם.
שֶׁכָּל פָּחוֹת מִשְּׁתוּת דֶּרֶךְ הַכּל לִמְחל בּוֹ: "
"If the fraud has amounted somewhat less than that, — — the defrauder is not required to restore anything, since it is generally customary to forego anything less than a sixth. "
This "customary" - "דרך הכל" seems illogical to me, would you agree to forgive 17%? For example, imagine buying a siddur from a local synagogue's Gabbay for $70 and a day later stumbling upon an official pricelist that states that the price is $60. Would you feel as "surely forgive"?
THe question: was it a common practice in their times that's backed by external sources (maybe Greek or Roman law) or what's the explanation of that (nonexistent now) psychology?