I would venture a guess than anyone describing their situation as you did would experience the same thing. In order for your sites to "stick" in the index, they must be "live" before the end of the current "dance" in order to be listed next month. So, the index you are seeing today was created with spidering data created at the end of August.
Your sites appeared temporarily due to "fresh" spidering during the month. I don't pretend to fully understand it, but if Google finds fresh content during the month, it often appears temporarily in their SERPs. There are many threads here on WebmasterWorld describing your exact situation, and some of them may contain better quailty info than what I have posted here. But the situation you describe is "normal", and if you continue to get spidered, your sites will appear "permanently" after the next cycle.
If you put up a site on exactly the right day, then it would take a month to get into the index permanently. If you miss by a day, then it takes up to two months. In the meantime your sites may bounce in and out of the index as Google continues finding and including temporary "fresh" listings.
You will likely see Googlebot come by for a deep spidering run within the next week.
Hang in there...