I think it's the light (and shadow) of the pic.
It's likely a result of the difficulty of even getting light in there and getting a good shot.
Even pushing as hard as I can, the screwdriver just climbs smoothly out and across the top of the screw. I'm using what I think are hollow ground tips in my megapro screwdriver. They're not as hard as I'd like though, I've twisted both of my blades today! This machine is so unwilling! I smacked the screwdriver a few times with my hands, no luck. Then I smacked the end with a wrench, still nothing to grab. My MegaPro is my go to when stuff just doesn't want to come loose and it's not making the grade today.
I also put some tri flow in there to marinate and hopefully have a good "talk" with the set screw to let it see the error of its ways. Nothing yet.
I've also had to do a similar operation on -both- of the brush caps because they're both broken off flush with the motor.