Basically the reasoning is this:
There are usually 2 non-NK neuts (counting king and not counting merc)
that means that in most games its 14 faction members and to start:
2 of them are Unseen/Cult
1 of them is NK and
11 of them are BD
In most games there would be an average of 4 converts (this number might have changed. Someone please check to see if this is true. It tends to be in my experience. You should count mercs on evil in this number) for a total of 6 Evils (unseen/cult) 7 BD and 1 NK
Now if everyone is exactly as skilled as everyone else that means that wining is determined purely by random factors (this would never actually happen because it is highly unlikely for everyone to be the same skill level).
There are 2 ways you can randomly assign a winier that are relevant to this.
1: Pick a random player and that players faction wins. With that mindset the winrates would be 50/43/7
OR
2: Pick a random faction that wins. The winrates for that would be 33/33/33
To allow for the benefits of both methods I averaged the 2 values to get 41/38/20/1
I had the 1% draw because there should be the possibility for everyone to die.