It's when you're idling it from a cold start mainly, because the mixture is richer and the engine takes longer to warm up so during this warmup time the rings don't seal as well so the fuel makes it past the rings and into the oil.
I didn't even idle my car for very long this past winter (most of the time I'd just start and go but if the windows were badly fogged up I'd sometimes have to idle it a minute or two first) and I saw 1.0% fuel in my oil. In the warmer months I see 0.0% fuel in the oil.