It's called market segmentation or something like that. nVidia produces one very configurable chip and then sells it in different configurations that essentially have the same elements and hence the same bill of materials to different market segments, although one would usually expect to find elements of higher quality on the more expensive Quadro boards.
What differentiates Quadro from GeForce is that GeForce usually has its dual precision floating point performance severely limited, e.g. to 1/4 or 1/8 of that of the Quadro/Tesla GPUs. This limitation is purely artificial and imposed on solely to differentiate the gamer/enthusiast segment from the professional segment. Lower DP performance makes GeForce boards bad candidates for stuff like scientific or engineering computing and those are markets where money streams from. Also Quadros (arguably) have more display channels and faster RAMDACs which allows them to drive more and higher resolution screens, a sort of setup perceived as professional for CAD/CAM work.
As Jason Morgan has pointed out, there are tricks that can unlock some of the disabled features in GeForce to bring them in par with Quadro. Those usually involves soldering and voids the warranty on the card. Since HPC puts lots of stress on the hardware and malfunctions occur more frequently that one would like them to, I would advise against using cheap tricks.