I think this is a simple misunderstanding of what is visible in the images.
1) There is some black marring (tarmac oils/track rubber) down the centerline which gives the illusion of the wear only taking place along the center line. However, if you look closely, the entire plank is worn. Look at the curved edges near the bolt holes (yellow arrows) on the outer edge of the plank on the the right side. Tricky to spot in 2D photo but the curved edges are the result of the plank wear on left and right edges.
2) If you take a piece of wood and fasten it down at several locations with bolts, the area in the immediate vicinity of the bolt holes will be "sunken in" due to the clamping pressure of the bolt. As an example, look at the stress pattern arising in a metal plate from bolt clamp pressure:
The area in the immediate vicinity of the bolt is inset due to the clamping pressure. That's why there are "light patches" immediately in the surrounding area of each bolt hole. This is particularly visible down the left hand side of the floor. These circled regions (blue circle) do not wear as much because they are slightly above the reference plane due to deformation from the bolt pressure.
3) No track surface is perfectly flat. So even if the plank is perfectly flat, it will not skim the circuit in a perfectly flat manner. Furthermore, even if the track was perfectly flat, the car has a non-zero pitch angle meaning the plank will not fully contact the track surface in a uniform way. The car is always at some non-zero pitch and roll angle due to bumps, suspension movement, and so on. Uniform wear is an impossibility.