If a Carbon Monoxide alarm must alert at 35 ppm, how can it randomly go off all of a sudden at 150 ppm?

I know carbon monoxide alarms must go off at 35 parts per million, so how can a carbon monoxide alarm go off for the first time all of a sudden and read like 150 parts per million? This is not happening to me, but I learned about CO alarms in a cl today. I am wondering how a CO alarm can skip 115 ppm though? Anybody know how these things work better than I do?