It could be that you knit tighter than you purl, or vice versa. If you purl loosely, then the st st on the front will look loose, or look tight if you purl tight. Take a look to see what stitch you're doing for the looser ones. Maybe you need to use a smaller needle to purl? It's not that unusual, I've read that people have had to do this before.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry