Future Market

Playing support and using future market as a rune and whenever I buy something using the last remaining gold i have calculated between the little gold i have in + and using the "-" gold from future market i always go on a greater "-" gold that I should go. For example if i have -100 gold and my max "-" is -175 and i purchase a control ward I should go to -175 (I have -100 gold control ward 75g = -175g) but I go to -200 or -210. I've seen this bug on numerous occasions while buying different items.