A roofer in a gray sweatshirt and orange hat walks on a unfinished roof at a lake-side home.
The man is fixing a bike.
It would be very odd for a roofer to be fixing a bike on the roof.
The man cannot be fixing a bike and walks on a roof at the same time.
The first sentence is about a man who fixes roofs for a living. The second sentence is about a man who knows how to fix a bike. Two different actions.