I teach the Kaplan course and I wouldn't call the PAT a joke, but it is a bit easier. In general you'll find:
Keyholes: Kaplan slightly easier.
TFE: About same difficulty.
Angle Ranking: Real test is way harder.
Hole Punching: Real test is way harder.
Cube Counting: Same difficulty.
Pattern Folding: About same difficulty.
I also think the Kaplan scoring system is off on the PAT. I think you can miss a lot more questions on the Kaplan tests and still score a 20 than on the real test. Not positive as you don't actually get to see how many you've missed on the real test, but I didn't find the actual test that much more difficult than Kaplan, yet received a 23 while getting 28s on the Kaplan tests.
That being said, a 21 on the Kaplan Midterm is really good assuming the diagnostic was your initial exposure to the material.