Some reasons the CEL comes on take a while to collect data before deciding there's an error. Therefore, there's a "readiness" code set, basically saying it's monitored long enough to give an accurate readout. OTHERWISE, people would reset the light and immediately get the smog check done before the light could come back. You wouldn't want that, would you?
I don't know the exact algorithm used to decide when to set a readiness code. But it's both start/stop cycles and miles, and I'm pretty sure less than 150 of 'em.