I have finished the sweater, including the dreaded sleeves. When I did the matress stitch one one side seam, it seemed I had three rows too many (I counted after the fact, not as I went- next time, a row counter!).
I sewed the opposite side seam just to make sure I was in fact three rows off, before ripping out and redoing the bottom third of the sweater (I can’t just rip out 3 rows because of the 8 inches of ribbing at the bottom). The second side seam was only one row off which I understand you can fudge when doing matress stitch.
Now quite perplexed, I redid the first seam, and was three rows off again. I am being very careful, it is indeed 3 rows. I don’t know how this is possible, but I’m going to rip out to above the ribbing and redo it minus three rows. Wish me luck.