Can someone help me understand these vehicle smog test results (details inside)?

In California, we have to put our cars through a “smog” test every other year. In this test, HC (hydrocarbons), CO (carbon monoxide), and NOx (nitrogen oxides) emissions are measured (among other tests).

So, my vehicle passed, but just barely. But here’s what I find a bit strange:

At 15 mph or low rpm, HC, CO and NOx emissions were close to the maximum allowed. But at 25 mph or high rpm, these same emissions were WAY below average. So, I’m wondering how could this be? Could it be that the fuel mixture is too “rich” at idle, or is this irrelevant?

I’ve also heard that you should drive your car around for a while just before taking it in for the smog test. I forgot to do it this time. Maybe that explains the results?