As I understand it, the actual CPC you pay is the minimum amount you would need to pay to obtain the ranking given for your keyword. This will depend on the other sites bidding for you keyword and could be less than the assigned minimum bid for your site.
Another possibility is that you are receiving clicks from the content network. This can still happen even if your bid is below the level needed to enable your ad to show on the search network.
You mean some keywords are inactive because your bid is below the min bid? Inspite of this you are getting impressions and clicks? Yes, it happens sometimes.
Maybe you are loking at avg. figures which include content + Search figures . This can be much below the min(Search) bid. It frequently is, for me as I bid much lower for content than for SEARCH. Even if you donot bid seperately, you will not have to pay as much for Content as you do for Search.
OK? Let's try this again.
Avg CPC for keyword.. OK? With me?
Min Bid for same keyword? OK
I have a situation where Avg CPC < Min Bid. Min Bid has never been raised.
There's a possibility that the situation you're describing is happening for the nicest possible reason: your ads are better than average so the ranking formulas are giving you credit for a better clickthrough rate than your competitors. That can reduce the amount you have to pay per click.