Years ago, I drove the pass in a rental 4WD Explorer. The highway patrol limited traffic to cars and trucks with chains or 4WD vehicles.
It was snowing lightly when I made the trip down into California. Sun was shining when I arrived.
I noticed that phenomenon while on a ski trip to Utah. While the requirement was never in effect, there was a sign at the mountain rd. junction which explained what you had to do when the warning was in effect. There was even a parking lot just down road a bit which was used to put your chains on.
Remember decades ago when studded snow tires were the norm?
When I lived out there, you always had chains in your car. Chains on 2wd and carry chains for 4wd meant it was passable, but snow on the road. If they required chains on 4wd vehicles that was code to stay at home.