Emissions ppm

I had my '92 metro 1.0 emissions checked again this year and barely passed on the first try. (216 ppm at idle - 220 ppm limit). I'm good for another 2 years, but am curious. Last time I failed the first test, timing was set at 10 BTDC and idle was set about 950 RPM. I re-set the timing to 5 BTDC and idle to about 875 and passed by a good margin. This time, timing was set at 8 BTDC and idle about 900 and barely squeaked by. Any ideas on which (timing or idle speed) made the difference last time? I've always thought that a motor ran less efficiently at a slow idle that at a faster speed, and earlier timing also helped efficiency. Apparently I was wrong. Interesting, when I was getting 50+ MPG, the timing was set at 10 BTDC and idle speed was around 950, which would indicate good efficiency, but yet it failed the smog test. I'm stumped. Could someone please enlighten me as to why I was getting great gas mileage, but still had too much unburned gas going out the tailpipe?
Thanks,

There are about four different numbers that they measure, and they mean different things. The ignition timing was retarded to 5° specifically to lower NOx emissions, that late timing increases unburned hydrocarbon emissions. It's kinda a dance, you can adjust a little each way depending upon which one your particular engine is having trouble with. Advancing the timing to 10° improves everything except NOx, better mileage, more power, longer valve life, less HC.