Well you must've had the bewitched versions of Mac OS X upgrade discs. They don't ask you anything, they auto-check for a previous install of Mac OS X. If it's not there (such as you installing a brand new empty drive) a message will appear saying something to the effect of, "This version of Mac OS X cannot be installed on this volume, no current installation found".
The only discs that qualified for what was referred to as an "upgrade" disc was the up-to-date discs or the drop-in discs that were placed inside boxes of new Macs that didn't come shipped with the latest OS on the drive image. If you had any of these discs and they did not perform a check for a preinstalled version of OS X then as I said earlier you had the bewitched versions.
You're certainly welcome to doubt all you want but tell us please, why do you think Apple said the $29 version was for Leopard users? If it didn't perform a check then why would Apple require you to be a Leopard user? Answer that if you will.