This is normal; in fact, it's built that way for a reason. The drive is not designed to teeter from side to side during operation; it must rest solidly on the camwell surfaces. In order to account for manufacturing variations in either the Drive or the well, a front to rear gap is designed in. Accordingly, some drivewells will rock more than others; it's not important as long as there is no lateral rocking.
Remember, the Drive is really supported and reinforced at the cam bolts and the front of the drive well, where all the pressure is applied. If you think about it, there is no substantial pressure on the rear of the drivewell -- it is simply an alignment slot.
