SD Card Drive question

I have a general question about how windows assigns drive letters. My son has a Dell laptop with Windows 7. First of all, the card reader works fine. However, I thought the drive letter was always fixed for the sd card reader. When he inserts a micro sd card and adapter in the slot, it pops up the windows info box with the header stating Secure Digital Card F:/. All is well. When he inserts a different micro sd card and adapter in the same slot, it pops up the info box with the header Secure Digital Card G:/. I don't understand the different drive letters. These are identical 2gb micro sd cards. The reason I ask is we are writing an application that stores the sd card drive in a local sqlite database upon initialization of the application. If the drive is not consistent it basically messes up our application. Any info on the multiple drive letters would be appreciated.