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When you pick up sts to knit the gusset, you're picking up in the slipped stitch edge plus maybe an extra stitch or so at the ends.
The slipped sts span 2 rows so you're not really picking up a stitch in each row but in every other row.
There should be slipped sts along the heel flap and at the turn. That's about 10-11sts picked up along the heel and the rest of the 15 where the heel turned. See if this video helps with the idea of the picked-up gusset sts.