You seem to think the only two options are they're lying or they magically know exactly what they experienced and should be trusted implicitly.

There's a lot more to it than that. People hallucinate and confabulate. People see patterns where there are none all the time. People unintentionally modify their own memories in all sorts of ways whenever they recall them. People believe things with no evidence just because they want to. People embellish their stories to fit the conclusion they want.