Why do different renders of the same project sound slightly different?

[spasticmouse]

I am rendering 320 kbps MP3s of a finished project and I'm noticing that certain parts of renders that should be identical sound slightly different. For example some notes are popping out of the mix a little more in some renders than others. I've noticed that it's not only about the render, but that each time the project is played it sounds slightly different. Anyone have any idea what this is about, and how to keep the project sounding exactly the same each time it's played?

[Michael Dow]

What sounds are you using? Can you do a null ...

[Igro]

Analog behavior of some vst fx and instruments ...

[Alphons]

Most common culprits are continuous running osc...