Honestly in my experience it has to do with your driving history when they look it up and your age. If you are a kid or in your 20's (looking like a punk, like so many kids in Southern California) then you have a higher chance of getting a ticket. I got them all the time. I had a couple speeding tickets before 1995 and would just tack them on.
Now in my 30s they don't bother and the 2 times I have been pulled over here, not one mentioned no front plate.