I hear ya! I was blessed with a calm 3 yr old paint mare that was ground broke but had never been ridden. The first time I got on her I was expecting a blow up but it never happened. I've been training her since last summer and I have yet to fall off (knock on wood). I think the main reason why she hasn't bucked is because she's lazy and doesn't want to make the effort to expend any extra energy. I'm thankful for my lazy horse because my old bones couldn't take the fall!
So true that some babies are dead broke from the start and never do anything and then there are some you start the same way, do all the same things, and they decide to unload you after a few rides. Maybe those are the smart ones!