Could you explain
Working in St st, inc each side every 3rd row o times, every 4th row 13 times, and every 6th row 0 times. Work even until piece measures 16 3/4 inches from beg , end with ws row. I did this and the sleeve measures 21 inches and I haven't even begun arm hole shaping. I knitted 3 rows and 4th row made the increase then knitted 2 more rows, then continued in this manner 13 times.