This dec row seems like some shaping which is later repeated.
“Dec Row 1– K 10, K 2 tog, work to last 12 sts. K 2 tog, K 10. Work even until 1st row of next Garter Stitch stripe is completed. Next Dec Row– Repeat Dec Row 1.”
Knit 10 sts, k2tog, work across in pattern to the last 12, k2tog, k10. Work even means to to continue in pattern with no increases or decreases.