Site Based ads (ads where the advertiser requests to displayed on your site, not always keyword based, people that click that advertise on this site link are site based) are impression based. As was stated earlier, image ads are impression based. The effect of the 1 cent/ 1 impression was most likely caused by an overflow from previous days. If, for example, you had generated .49 cents (that's 49 tenths of a cent, not 49 cents) from impressions on days prior, then that first impression would bump your total to a roundable number (I have no idea where or when Google rounds, you may actually have to hit the penny mark).