For a couple of reasons. First, the German fighters were seeking out the bombers, not running from them. They were getting closer, if only for a brief second.

Second, the B-17s had an enormous amount of firepower, seemingly unlimited .50 caliber stuff, which is heavy and lethal.

Third, it wasn't "mano a mano". When a German fighter approached a bomber, there were bombers all over who could take a shot at him.

And I can imagine an 18 y/o waist gunner with a German fighter more or less in his sights following that plane behind the B-17 next to him. The really did shoot down a lot of themselves.

Not something which is publicized, though. I can see it now.

A B-17 with 3 German planes on it's nose, and 2 B-17s, the crew claiming to be aces.