She said twist, not curl. Remember someone posted a picture here a few months ago of their garter stitch scarf/tie twisting?
As far as it being the yarn - when yarns are spun, they're twisted and if you knit opposite the twist they can twist back, causing the whole piece to come out weird. But if this happened with all of them and you're using different yarns, that's probably not it. 20 stitches should be wide enough so
I dunno. Maybe you are knitting too tightly.