Mostly in general, but I listen to the Boston ATC a lot. Right now I'm listening to Boston Aug02 on 134.95 and only hear the controllers.
1) Receiver is out of range of the ground side. It doesn't happen often but sometimes setups degrade (water in coax, antennas fall down, etc.) over time.
2) You are listening to a frequency that is being simulcasted on. Your example in particular, 134.95, especially during the day, is a Boston Center (ZBW) frequency that is being "worked" by a single controller who is on several frequencies at the same time. You will only hear pilots reply on that frequency if they are not in one of the other sectors being controlled at the same time. The controller broadcasts on several frequencies and listens on them all as well. When traffic starts to build up, to avoid confusion you will hear the controller say: "Change to my frequency, xxx.xx" in order to get most of the planes on one frequency.
Hope this helps explain things. Ask away if you have any other questions.