Hmm good question. The bullet setting adds more steps in the physics engine, so it's probably simply that more steps = more chances that when the collision triggers in Construct there also happens to be a collision in the physics engine.
That makes sense.
Without trying I'd suspect though that it's still not 100% reliant because the physics engine treats the shape as a perfect circle whereas Construct just uses the collision poly.
No, it is not 100% reliant.