The twist of the singles is going to "move around" after you have skeined the yarn, any larger bumpier sections are going to move the twist to the thinner areas, which end up with more twist.
Idealy for singles you shouldn't need much tension on it unless you are using the yarn for weaving- if you are using a lot of tension your yarn is going to spring back when it is blocked again after you knit and won't have the stretch desirable for most knitting yarns.
To get rid of the extra twist, if that is your problem, sometimes people find that it is helpful to go back through and unspin the yarn ever so slightly. Or of course, ply the yarn.
To help distribute the twist in the yarn:
After you have finished spinning, wind the yarn off in to a skein. Soak the yarn (10-20 minutes so it is nice and wet). Then wring the yarn out slightly so you don't get water everywhere and thwack it against the bathtub, sing, outdoor railing, etc This can help convince some of the twist to move and distribute itself correctly in the skein.
Also, steam the yarn (I hold it over a teakettle, boiling pot of water, etc for a few minutes) to help the yarn bloom- this will help the thinner sections bloom and fill in like the thicker sections.