That was the curve the developers were comfortable proposing at this point. There are some key properties that the reward curve needs to satisfy and you can't easily just pick any shape you want without causing problems somewhere else. It can be updated later, but one thing to keep in mind is that we won't really even know if 20 is the right break even point until after the new system has been in operation for a while. The numbers 16 or 20 come from running the existing votes through the new curve but that is nonsense because people will certainly change voting practices, there will be more (how many more we don't know) downvotes pushing rewards around, etc. So we'll see.