You're in San Diego and if I'm not mistaken you have the Dyno smog test (Smog check II). The dyno should read correct speed I would think. Should still be doing the higher speed test first, followed by the lower speed test (non dyno area testing is high idle [2500rpm] followed by low idle [800rpm])
A bad, or aging O2 sensor will not allow the mixture to normalize quick enough to pass the smog test. Possible your O2 sensor needs replacement.
Also good practice to drive for an extended period before taking it in for the test, to have the Cat good and hot and fully up to operating temp. This will help get a marginal Cat through one last test as well. I usually drive any car I'm taking in for smog at least 1 to 1.5 hours then leave it running while waiting for the test. Have not had one retest in the 17 years I've been in California.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.