The cover doesn't bend or deflect when you overtighten.
When you overtighten, you squash the perimeter seal and the whole rocker cover is closer to the rocker arms.
That makes sense if it were true. HOWEVER... it means that Toyota designed the cover to rockers distance to a ridiculously tight tolerance. Someone will have to show me that the cover, without the rubber seal gasket, just sitting on the head, the inside top of the cover baffles will contact the rockers. The distance, or gap between the bottom of the cover and the head deck with a new gasket, grommets, and acorn nuts properly torqued to spec is about .020".. just visual guess. When I pulled the cover, I looked carefully at the baffles inside the cover... no evidence of anything touching it.
The next time I have the cover off, I will carefully mic the distance between the base of the cover (gasket grove) and the highest point from the head deck to the rockers.