From what I understand so far, a hero's net worth grows over the time of a game. The value seems to include the hero's inventory and its current gold. Therefore I get that position 1 and 2 players usually have a higher value than those on position 4 or 5.
I just heard them say on stream, a really underfarmed support had a lower net worth after five minutes than he had at the start of the game. (Winter Wyvern in this screenshot)
How would that be possible?