Confused with knitting pattern instructions

Hi there everybody.

Please can you help cause i'm sooooo confused!

I'm knitting a jersey and am busy with the left front. it says to work evenly until piece measures 12.5" from beg, end on WS.( I've got 43sts) Then i enter the Inc Row which says: K1, M1k work to end (44sts). Rep this inc every 1" 3 times more. AT THE SAME TIME (?), when piece measures 13.5" from beg, end on WS.

No my question is, if i'm on 12.5" and then i start increasing one stitch every 1" 3 times surely it will be longer than the 13.5" it says i should have?

Or am I reading it wrong?

Thanks so much