I know the main ranking list is the one shown when the game ends, but it's bothering me why some heroes almost always have the most unit production value. Since no one can really create units in DotA, what does it represent?
Unit Production is just a hold-over from the fact that this is a Warcraft 3 map - the stat is standard in the normal game. It does have a disproportionate impact on the end rankings, though, so it does heavily skew the results.
Certain skills of heroes in DotA are performed by essentially creating a whole bunch of units for a limitted time. For example, Dwarven Sniper's Scattershot (if he still has that, it's been a while) creates a lot of units in order to produce the effect, so he tends to be one of the "high unit production" heroes. The logic behind it varies based on the hero and the skill in question, but if you look at which heroes tend to have high unit production, you can usually figure out what skill is the cause of these rankings.