Confused by instructions in paranthesis "K1(2:1:2)

#1

I want to make a sweater from “Adorable Knits for Tots” by Zoe Mellor. The pattern is the cover pattern (little star sweater) and is for sizes 6-12 mths, 1-2, 2-3, 3-4 years. I’m not sure what size the instructions are for. The first rib row instructions are K1(2:1:2), p2, [k2, p2] to last 1(2:1:2) sts, k1(2:1:2).

At first I thought the instructions in parens were for the different pattern sizes, but why would the next size up progression go up, then down, then up (2:1:2)?

#2

The numbers in the parentheses are the size progression. Don’t worry about the numbers of stitches in there, the designer has taken all things into account, and probably the cast on, or bind off, or increase/decrease sections compensate.