Not true.
Apple has historically had the stock versions, plus what they internally call an "ultimate" version, which can have the next-best video card, a bit more RAM, and occasionally a larger hard drive.
The target for in-store ultimate configs was the pro laptops and desktops. But, for historical note, the iMacs used to have a config like this when there were only two options for the 20" (when 17" and 20" were the two options.) BT/AP have long become standard, and the hard drives are already maxed to 1TB in the top model so it is unlikely for the iMac.
In the (recent and distant) past, stores have carried ultimate configs for MacBook Pro (17" and 15" had maxed out hard drive, video card, and RAM); and Mac Pro (upped video card and RAM.)
I would expect to see a Mac Pro ultimate model in stores with the ATI card and 8GB of RAM, but not for the 4-core version. Apple tends to only use ultimate configs with the top-end computer in the class.