I asked one of my driving instructors about that, and he said, "Well, they probably don't know what's being tested. For instance, on your test, when you make a lane change, you have to check the rearview mirror, the side mirror, and then look over your shoulder, in that order. If they don't know that, they'll get points taken off, and there are a lot of places like that that they might not know. When I got my license for specialty vehicles (e.g. backhoes, construction vehicles, farm vehicles), I had to take the test six times."
One of the car salesmen I was talking to also mentioned something about tests, so I asked him, "Is it very hard to pass the driving test at the test center?" He said, "Absolutely! It's much easier to pass the test at a driving school. Those test centers are tough. People who go to the test center usually have to take it about 3 times before they pass!"