Why do undue cards always pop up during Study sessions? I only did 4 new words and it still started showing me undue cards...

I just renewed my subscription, but I now remember why I stopped last time.

Maybe there is something I am missing?

You can see cards that aren't quite due if you don't have many items due in total, or if there's a bad internet connection and the server and client get into a short-term disagreement over what's due. "Due" is a rather arbitrary threshold. Studying something that's 90% due is almost as good as studying something that's 110% due, for example.

If it's not adding new words fast enough, you can look here for some options: http://www.skritter.com/faq#more_words