Initial calibration is a quick best guess. It cannot predict with 100% accuracy every word you may or may not know. One option, the IKT--or "I Know This"--button, gives you the choice to skip a word the first time you see it. This article will explain some of the ins and outs of the IKT button.
When You'll See the IKT Button
The IKT button always appears next to the headword in the upper right hand corner of a new word page.
The IKT option is presented only when you see a new word for the first time, so you only get one chance to use it per word. IKT should only be used on words that are known well--if there are any doubts at all, the button should not be used. IKT words will be removed from your training stream, so you will not see them again. IKT words are tested on quizzes to ensure that they are actually in a your working vocabulary, hence skipping words via indiscriminate use of IKT is not a good idea!
Unmarking Words IKT
If you've accidentally marked a word as IKT, you can easily undo this. From your dashboard, click on the "See and Undo" link beneath "All Learning Sessions."
You'll be taken to the list of words that you have skipped. Check words that you want to revert and hit "Undo IKT."