help with Rowan Hiker sweater pattern

I’m getting read to shape armholes and divide for front opening and I have a couple questions – a- I have 77 stitches after casting off 2 at beg of next 2 rows.
Next row, K2, sl 1, K1, psso, K 31, P1, K5, M1 and turn leaving rem sts on holder. That means I’ll have 40 stitches to work on and 33 on the holder? (it says to work each side separately? why would it be off center like this

Welcome to the forum!
It’s likely that it’s because some of the center sts will become part of the center placket. Further along in the pattern it will tell you what to do with the center sts. Probably there will be directions to cast on some sts and knit them together with the center sts to overlap the placket.

Reading further in the pattern may help clear this up.