"Can someone help me understand what this 'clever mounting' can be ?"
This is my understanding of one way to "change" that dimension, and potentially in TD 039 shakeout.
If the plank was mounted, in it's entirety, to the floor with a substantially inflexible "packer" layer, but the areas around critical measurement holes were of a more "compressible" (we'll call it) substrate, then impact with track would see the overall plank wearing on such a strike .....while the reference material and its area was compressed into its supporting substrate.
Measurements after race will never look at the overall plank material left in place, just the specific reference material at those critical sites, which could have moved under hard impact with track. They test for this movement with static support test, from my understanding. But like wing flex, that's unlikely to be finite in its depth of scope.
I don't know what they changed exactly in absolute detail for 039, but you can see in reading the current notes, back in this thread, that the reference plane, fixings, how those fixings are placed etc, are all defined in absolute terms that to prevent "interpretation from the teams, naturally.
