Listen to Panacea and read _Google Information for Webmasters_ at [google.co.uk...]
It clearly states:
Each time we update our database of web pages (about once a month), our index shifts: we find new sites, we lose some sites, and site rankings change. If your site was dropped from Google and you have not made major changes to it in the last month, we will likely pick it up again in our next index. It's possible your site was simply inaccessible when our robots tried to crawl it.
Unfortunately, the reality is even worse than this admission.