Take a look at the screenshot with your ads:
You have 4 active ones and 4 sold/expired ones. Two of the sold ones are from yesterday, so as of yesterday you clearly could post up to the full limit of 6 active ads. I don't know when the two ads you just sold would stop counting just as the other two (from Sept 7 and Sept 16) clearly don't count, or how long the ones you have active are going to run for (you may have selected really long period).
I don't see an ad that was sold over 20 days ago which you say is counting.
I suppose the easiest way of taking an item off the market is to just mark it as sold. You could also lobby the software makers to start distinguish again between sold and expired ads.
We really would like to keep the sold and expired ads forever in the archive section, but since the software seems to delete them, for now they just stay as long as possible.