My ability to add short columns of small numbers is admittedly suspect, but I count 18 too.
If you're getting 20, I'd guess the most likely suspects would be in the YO's getting an extra wrap. The decreases are ... possible I guess, but it'd be hard to make 20 that way unless you just forgot to do them altogether. The YO's seem more likely since they happen on either side of a purl. Having to get the yarn to the front and still make the YO can be... confusing. At least it is to me. So it'd be easy to get an extra wrap in there I imagine.