Am I getting scammed at the smog test station

Hello, I have a 1995 Corolla (1.6L, 150,000 miles)) and it failed the CA emission smog test (Failed the Nitrous Oxide/NO test at 15 MPH/Passed the NO test at 25 MPH). Smog tech said I should have run the car for at least a 1/2 hour before bringing it in to be smogged so everything was HOT (oxygen sensors, catalytic converter) because the car just barely failed. Put in some better gas & drove it an hour at 60 mph and drove back to the test place (kept the engine running until the guy took the car into to be tested. Now the guy said it failed again, but his time because the EGR valve failed?? He said the diaphragm in the EGR isn't working. Sounds plausible except that the car runs PERFECT!!! And unless I am correct, if the EGR diaphragm really wasn't holding vacuum, the car would run like crap (it runs great) - or am I wrong. And I would assume that the car would fail by a LOT, and not almost pass. Is this possible or is the guy scamming me? Or maybe he has no clue. The NOx reading at 15 MPH MAX is 504PPM, my car 635 PPM. And if the EGR was really bad, wouldn't it fail the NOx test at 25 MPH as well? (My car passed at 25 mph) Anyone know?