Monitoring seems confused

Of course, it is not QTractor's monitoring that is confused (I assume), but I certainly am.

Mostly to try to learn how to use QTractor, I have been trying to record myself playing a trio. (I also have a MIDI project I have been experimenting with.)

I recorded a guide track, and then recorded each of the 3 instrumental parts, listening to the guide track and/or the previously recorded instrumental track(s) if any. I was using an earpiece to listen to the previously recorded material.

Unfortunately I could always hear what I was currently playing through the earpiece, which made the whole thing very difficult (and not very successful). Eventually I gave up in despair!

Later on I reloaded the session to have another look at things and try to sort out the problem, I discovered that I had "Auto Montor" activated. Once I turned this off, it solved the problem.

All the same, I don't understand this monitoring business at all. I have my microphone connected to QTractor's Audio Master In, I have 4 audio tracks, and I have QTractor's Audio Master Out connected to System Playback. This has to be correct, I'm sure. However, I find that if I click on the Monitor button of any of my 4 tracks or either audio bus, then the input from my mic is relayed through my earpiece. There seems to be no sense in this (to me) - why do we have all those buttons if they all do exactly the same thing - feeding Master In to Master Out?

Clearly I'm misunderstanding something here, but I would be very grateful if someone could explain it to me. I'm a retired musician and I know a lot about music, but little or nothing about sound engineering, so I will undoubtedly have a few more of these apparently daft and elementary queries as I go along. But we all have to start somewhere!

David