I think the main thing to understand is that total working load on the bolts holding the main bearing cap is smaller than the total load acting on the cap.
So for example say you have a 2 bolt cap with 50 kN acting on the bearing, the actual load per bolt (excluding existing pretension) would be much lower than 25 kN. That is because under external loading the joint behaves as 2 springs in parallel. One spring is the bolt, the other spring is the main bearing panel. The joint force is distributed according to the rates of these 2 springs. A well designed bolted joint will have a bolt which is much more compliant than the section it is clamping so it will take much less force.
If you were to use the same bolt in an aluminium block and in a cast iron block, the bolt in aluminium would see higher working loads since its relative compliance to aluminium (E~70 GPA) is higher. Cast iron E is about 100 GPa so it will be stiffer with respect to the bolt.
Have a look here, from p51 onwards it tells you everything you need to know: