Where is this idea that fights last three rounds coming from?

Whenever I'm designing homebrew I get the same refrain repeated over and over, remember that most fights last three rounds so balance around that.

But since when is that the average? Sure, some can be that short. If you start 30' away from the other side with no obstacles and neither side tries to crowd control, it's not going to be a long combat. But between things like distance, terrain, control, alternate objectives and reinforcements there are so many things that can end up dragging out fights considerably.

I'm just not seeing where an average of three rounds comes from, and would like to know if that's just because my sample size only consists of my own games.