Reading HPT late

Elani • Been TTC for a year now (Feb 2015). 3 failed rounds of clomid. Hoping for the best, a pink one as I have 2 gorgeous sons
Maybe a dumb question -  what IS the deal with reading tests more than 15min after poas? Can it falsely give colour? Or is it about the evap that makes it look like a line, but with no colour? Can it color pink falsely, with no hcg hormone? Please advise as I'm dying to know the actual REASON why you should disregard results after 15min?