Because this is a multiple of 2 + 1 stitches. The below count should clear up any issues about borders.
If you cast on 21 stitches it should be broken down like this:
Rows 1-2 Knit across
Row 1 Knit
Row 2: K2, P17, K2
Row 3: K2, K1, *(YIB) Sl 1 (KW), K1* Repeat between * 8 times, end K2
Row 4: K2, K1, *(YIF) Sl 1 (PW), K1 * Repeat between * 8 times, end K2
If you want a dishcloth, I would recommend using 35 stitches for pattern and then add the border of 4 or 6 stitches.
Repeat the above 4 rows till desired length. And end with 2 rows of knit.
If there are further questions please let me know.