"With 6 mm needles, cast on 111 (123) sts and work dropped stitch pattern, rep rows 1 and 2, 6 times, then working row 3 once (= 75 (83) sts)"
I'm not sure, but it seems to me that as written you are only to do rows 1 and 2 (6 times) followed by row 3 once. Then you should have 75 stitches.
I don't see anything about doing all 6 rows of the drop stitch pattern. In doing so, you worked row 3 twice which resulted in the fewer stitches.