"What determines the number of Meltan spawns per mystery box?"

#PokemonGO: Usually I'll get 19, sometimes 18, and I'm curious if it's just random chance or if the rate of Meltan spawns is affected by anything. I'll find myself going to the encounter screen as quickly as possible and trying to catch them as quickly as possible, in case maybe the timer starts after catch, but that might be complete nonsense. via /r/TheSilphRoad http://bit.ly/2SlsWGB