Possible reasons:

Random test malfunction or miscalibration;
EGR valve stuck shut;
EGR passages clogged;
Timing overly advanced;
Exhaust leak before the cat;
Rich-biased O2 sensor.

Check the obvious: ignition timing and exhaust leaks. If OK, and if no
error codes stored by the ECU, then...

DON'T DO ANYTHING to the car. Book a retest in a week at a different
facility. Make sure the car has been driven for at least a half-hour at
highways speeds immediately prior to the test, and do not let it idle
before the test. Make sure you're on the dyno within ten minutes of
parking. If, and ONLY if, you fail the second time for the SAME reason,
should you start investigating the EGR.

Of course, if you don't mind tearing into th EGR yourself, then it costs
nothing to see if that's blocked before the next smog check.

---------------------------
Check and replace the Positive Crankcase Ventilation valve.

Often ignored. Oil will often last longer too.

'Curly'

This seems like a helpful site:
http://www.aircare.ca/index.php?repinfo-glossary.php . See especially
"Causes of Excess Emissions."

It puts a lot of emphasis on an improperly functioning EGR system causing
high NOx , but it lists a few other causes, too. None are the PCV valve.

http://www.interro.com/techgas.html#anchoreleven states similar.

An improperly functioning PCV valve may cause high CO, which you don't have.
OTOH, speaking as an amateur from my own car work and reading, I agree with
Curly that the PCV valve is commonly neglected. If it's never been replaced
before, consider doing so.