The drive doesn't care about the contents of the image: it's just zeroes and ones.
Test Mode, as the name says, is a test burn where everything goes like a normal burn except that data isn't actually written to the media. You can use it for any situation where you want to simulate the burn process yet you don't want to waste a disc on doing so (e.g. to check if your system can burn a DVD at 24x with a new drive). But you can only simulate so much...
Normal burning, though, is a much more complicated process from the hardware's point of view. For example: just before starting the burn process, the drive has to calibrate the laser power so that it changes the right spots on the disc. This is known as OPC (Optimal Power Calibration) and it isn't done in a simulated burn. There's also the problem of dirt on the lens, which affect the lens focusing ability and results in a lower quality burn, if it can burn at all.
I think you get the picture by now...