Are part numbers 12, 15, & 16, 1×2’s or 2×2’s?
The dimension on the cut list have 1.5 x 1.5 for these
pieces, and the material list calls them 1×2’s.
Later on in the directions it calls for 2×2 pegs inserted,
but the material list calls for no 2×2’s to be purchased.
The material list and cut list may use the nominal “callout” size, but in reality the finish size is always what matters.
It’s may be confusing, but you can thanks the lumber industry form making a product called a “two by four” that is actually 1.5″ x 3.5″.