and then there is multi-path cancellation.....which is exactly that, a signal arriving via different paths (one directly, one off a mountain, or filing cabinet, or whatever, arrive out of phase and cancel). Of coarse "multi" means "many", and that many signals all combine to do what they will at your antenna. BTW, although (for simplicity assume only two) if two signal arrive in phase, the power is only doubled, if they arrive out of phase they can cause complete cancellation...
Then we have capture of the reciever by a STRONG off frequency signal.... Attenuation is of cause in the equasion, but cancellation more likely. And on and on and on.....Move the antenna a coupla feet... The best answer to your question, wait for it,: "because". Ed