Row 3 has a decrease - k2tog, as well as an increase - knit front and back. And row 15 has the increase and a different decrease - ssk, which is the same as slip 1, k1, psso. With both on the same row, your stitch count stays the same. A slip stitch can be used at the beginning or end of a row, or in a decrease, or just slipped in the middle of a row and not worked until the next row, so it depends on the pattern.
sue- knitting heretic