It could be that they are not counting the 6 stitches for the placket. The pattern starts with 21 stitches for the front and then the placket stitches are added. My guess is that they were not included in the count.
"Trust the pattern!"
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Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry