It adds 2 at level 1 and that number goes up to 10 at level 5 like so:
There are some that only have 3 or 4 levels.
The base value is 1, so if there were 10 items in the list, each being 1 they would add up to 10, with each being a 10% chance. Adding 1 would make it a 2 out of 11, so now a 18% chance and so on. I think there are 23 magazines in the full list.
Your sample size of you vs them is not large, so randomness could be a big factor in what you saw.