What sources exhibit this behavior? Is it true for all of them, or just for certain ones?

An 18" difference in distance equates to about 1.5ms of difference due to delays associated with the speed of sound. That difference is very difficult to hear - some folks report being able to detect differences this small, but in general the rule of thumb I've heard is that it takes close to a 10ms difference for it to be detectable. That makes me suspect that if you are hearing a difference then the problem is an audio/video sync issue.