You can say it's a part of being human (something liberals especially, seem to be losing sight of in the present). There's only so much information the human brain can hold at one time, where you couldn't possibly expect to be 100% accurate in every little detail without meticulous fact-checking/planning done beforehand.
What separates Trump from the typical politician is he's not afraid to answer questions, speak his mind, be off-the-cuff; without any premeditation, orchestration, or teleprompter, to give off a perfect, clean response and image (and even those that do that, aren't 100% successful with it). I think Trump believes what he's saying to be true, simply because it's in his nature to speak in a spontaneous manner, without the over-calculation and concern for being wrong or looking like a fool. And it evidently has worked out pretty well for him up to now.