Why do people call it "flip flopping" when a politician changes his/her mind?

I know there are times when politicians do flip flop to get votes(John Kerry) and times where politicians say different things to different crowds(John McLame) but sometimes people get different facts that cause them to change their minds about things. When politicians do it, even if it happens over a period of 8 years it is still called flip flopping. Or in the case of Rand Paul they just speak realistically and realize they might have to take baby steps to accomplish what they want and know they can't do what they say over night. Why are people so fickle about this?