Yea, temperament. Even though we use the same 8-note system that was used during the "classical" period, the distance (in frequency) between certain pitches isn't the same as then, because we now (generally) use even-tempering.
Re:move away
Move away from today's temperament in pop music (or even how classical is played with modern instruments) and most people would probably be confused because of these slight frequency changes.
I've heard classical played with historically accurate temperament using instruments adjusted to try to reflect the sounds of the time - very different.
Check out prof Greenburg - pretty sure he does it in one of his lecture series.