Basic Trope: Despite changes in the past, improbable events still have occurred in the present.
Straight: Bob goes in back in time to kill Hitler before World War II and succeeds. He still exists, despite his dad's parents having met in the war.
Exaggerated: Bob destroys the asteroid that killed off all the dinosaurs. Humanity still evolves.
Downplayed: Bob creates a minor obstacle in the way of his grandparents' courtship. They still get together.
There are mysterious forces that try to keep the timeline on track.
Alternatively, since Bob changed history, he still exists in order to prevent a paradox
When Bob travels to other universes, he ensures his machine can only send him to realities where he exists. Since there's an infinite number of worlds, he could find a version of himself where the Nazis won WWII.