My suspicions lie with the Western League bringing up their Premier division to 20 by relegating Corsham and promoting Cadbury Heath, Calne Town, Gillingham Town & Melksham Town. That will take division 1 down to 16 teams and so presume it likely they'll take four new recruits, if there are four teams available to come up that qualify.
You are forgetting the "Buckland Factor"
So with Merthyr Town going up and Buckland Athletic coming in it'll be 18
Relegate Corsham Town and promote the top 3 in Division One = 20
Alternatively - leave Corsham where they are and sprinkle in a little Calne Town and you have 22 in the Prem and 15 in D1
Which would mean the Western D1 could take 3 promoted sides (one from each of the GSL (Cribbs), SCL (Cheddar) and Dorset League (Wincanton Town to make a 22/18 split