Pattern Body has two decreases at underarm, which results in one less stitch (for the 105 size) for Back section.
One extra stitch needed to be added to the Body / Back to have enough stitches to do the hem rib repeats.

I noticed this right before the 14th row of the Body and dropped rows of stitches down to back / underarm beginning area and removed a stitch that had been slipped over another per instructions, yet that created the issue of one less back stitch (Similar decrease at end of back section was fine to leave as is since that did not affect stitch count.). With 14 rows of stitches picked all the way back up, there were enough stitches to get the required amount of stitches as noted on pattern.

3/4 sleeves: 2 extra stitches picked up and decreased next rnd.
Decreases were k2tog & ssk (opposite than pattern.)
Following decreases were based on my arm circumference along the way.

6 repurposed buttons

All BO US10.5