I have a knitted cardigan pattern worked in 1 piece using a "Box St.Pattern".
1st row: (RS) *K3. P3. Rep from * to last 3 sts. K3.
2nd row: *P3. K3. Rep from * to last 3 sts. P3.
3rd & 4th rows: As 1st & 2nd rows.
5th row: *P3. K3. Rep from * to last 3 sts. P3.
6th row: *K3. P3. Rep from * to last 3 sts. K3.
7th & 8th rows: As 5th & 6th rows.
These 8 rows form Box St. Pat.

Ok, I understand all that, but my question is for the following statements:

Shape sides: Keeping cont of pat, inc 1 st each end of next 6 rows, taking inc sts into pat.

How do you keep the pattern, if you are only adding 2 stitches to each row? One at the beginning and one at the end of each row.

I know it increases 2 stitches per row, because the number of stitches which started at 105, went to 117.

I don't understand. Can someone help me figure this out? It's a really cute cardigan.