Pattern measurement help

I’m knitting the “Vestee” baby sweater on

I’m having trouble understanding their measurements.

Their gauge is 20sts/26rows = 4 inches in garter rib
For the back of the sweater they say to cast on 56 or 59 sts for the 12 or 18 month size.
According to their gauge, 56sts=11.2" and 59sts=11.8". Correct?

BUT, their finished measurements for the chest is 21" (for 12mo size) or 23" (for 18mo size). If you halve these numbers, you get 10.5" and 11.5" - which does not match up with my calculations according to the gauge.

Why the difference? Is it not supposed to match up exactly?

Thanks in advance for the help

It doesn’t always, there’s stitches for a seam allowance probably.