Does anybody understand, how this first page minimum bid works?

Yesterday I have just been offered a nice promotion gift from Google (via Webmaster tools) – a 100 USD of free advertisement. I have never yet used AdWords as our webpage is not really making any money so we do not have an advertisment budget. But, given the free gift, I decided to give it a try. First thing I had to do in order to spend this free gift was to pay 10 USD (to activate the account, which was 5 USD and another 5 USD for AdWords as it seems 10 USD is a minimum payment).
But, so far everything was quite OK.
I set up some simple ads and gave them a very low CPC (0.05 USD) – the reason was simple – the keywords I used to trigger the ad were to that point showing practically no ads at all.
It worked just fine. For a day. The ad was displayed and we received 117 visits through it, spending 5.48 USD.

Day two – out of nowhere, the 0.05 USD is not enough for the first page bid. I would have to increase it to 0.2 or 0.4 USD. It seems that the low quality score (4/10 – 6/10) affects the minimum bid.

So my two questions are:
-how come that 0.05 bid was OK for 23 hours and then not anymore?
-how come the minimum bid is too low and there are _no_ ads showing?

A thought crossed my mind that Google allowed me to spend those 5 USD from my credit card as I have wanted and now they will spend those "free" 100 USD doing a bit less work...

The statistics are available here:
http://users.cosylab.com/~gmilcinski/tmp/stats-20090823.png