"So is the reality of a hairline that looks like that unlikely with a hair piece? "

Well... it might not look that perfect every day, in all lighting to us.
But at the same time, in real life no-one else is EVER going to get THAT close and stare at it for as long as we can in a picture with a light source directed at it, at that angle.
So the general effect will be an invisible hairline to everyone we meet.
We, however, might be able to study our attachment in a mirror (for hours) and find imperfections.