It's magic. Mere mortals such as I aren't allowed the secret.
I think that the pattern is based on a given gauge for a given length of row. Sock yarns that produce a pattern would be based on rounds of knitting that will fit a typical foot. Not all multi yarns will produces the FI looking patterns, some are self-striping. How do they figure it out? Now I'm back to it's magic. I wonder what Google might reveal.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper