This description from the docs isn’t clear to me:
To solve this, you can enable continuous collision detection on the rigidbody of the fast-moving object. Set the collision detection mode to Continuous to prevent the rigidbody from passing through any static (ie, non-rigidbody) MeshColliders. Set it to Continuous Dynamic to also prevent the rigidbody from passing through any other supported rigidbodies with collision detection mode set to Continuous or Continuous Dynamic
I don’t know the exact description of it, but i understand it as
dynamic = better collision detection
continuous dynamic = even better collision detection 
It’s probably down to the physics polling times, it just polls more often with the continuous mode to counter 2 fast moving objects coming towards each other. Of course the performance impact will be increased as a result
Discrete collision updates the collider position once per fixed frame and detect collision between the overlapping colliders.
So if the object is fast enough to pass through the other in between fixed frames, it won’t hit. That’s why increasing the size of the collider helps too (although can result on strange behaves).
Continuous collision makes the object trace the route and detect the collision with static colliders even between frames.
Objects with this mode won’t pass through object with static colliders (without rigidbody), although it will still be possible for it to pass through another game object with a non static collider.
Dynamic continuous traces the route and cross the path with other continuous colliders looking for intersection, so it can make it collide even if there are two fast moving objects. This mode prevents one continuous dynamic collider from passing through continuous colliders, although it is still possible that it will pass through non static objects with Discrete collision.
Thank you for the explanation. 
You are welcome let me know of I can be of further help