Can't pass smog test

Well, I am no expert but here are the readings :

Test at 15 miles an hour : Failed Test
RPM 1597
CO2 % meas 14.80
O2 % Meas 0.04
HC (PPM) Max 88, AVE 18, MEAS 19
CO2 % Max 0.64, AVE 0.05, MEAS 0.09
NO (PPM) Max 508, AVE 149, MEAS 618

Looks like I passed the CO2 test, and the HC (PPM) test but failed the NO(PPM) test were they measured 618 where the max allowed is 508

Test at 25 miles an hour : Passed Test
RPM 1539
CO2 % meas 14.80
O2 % Meas 0.02
HC (PPM) Max 53, AVE 12, MEAS 16
CO2 % Max 0.78, AVE 0.04, MEAS 0.06
NO (PPM) Max 761, AVE 144, MEAS 550

So I passed at the 25MPH test but failed the 15 MPH test.
I passed all the emission Control System Visual Inspection/Functional checklist Results

any ideas why I failed the NO(PPM) test at 15 MPH?

Thanks