It may be something about this particular yarn that's emphasizing slight differences between knit and purl rows. i notice rowing out more with cotton yarn too.
I find that it's very difficult to be consistent when you try to purposely knit more tightly or more loosely. That kind of very even tension comes with practice.
It may be easier to try to adjust with slightly different needle sizes although the difference you're noticing may be too minor.
i thnk the above suggestions have just about covered every
thing mentioned here but maybe it helps to know that you're not alone.