My (limited) understanding of the lists.
At election time, we get 2 votes.
1 constituency
1 list
Overall, the number of list seats a party gets is determined by whether they get the constituency seats in the same region.
So Lothian area has a few constituency seats, 1 list. If SNP got all constituency seats, they'd likely get no list seats. If they lost 1 or 2 constituency seats, they'd have some 'fall back' from the list.
I defer to someone else explaining it better as I still get a bit confused!