They probably wanted you to cast on those stitches before you joined in the round. Backward loop would work for this. It's probably to help join the crotch stitches to the legs. I don't think I'd panic about it if you've already started down the leg. It's only 2 stitches, after all.
You can increase 2 stitches now if you want both legs to have the same number, or do the second leg the way you did this one.