OK, I can't keep my blabber mouth shut on this subject any longer.

molecular velocity is only a function of the temperature which is not changing in the diffuser. colder air has less molecular velocity.

The stagnation, pitot, total, dynamic pressure (Pt) does not change if we have a friction free wall.  Why does the static pressure (P) along the wall go up as we slow the air down? If Pt is not changing and velocity is decreasing then the Bernoulli equation lets us calculate the P value. We can ignore the small density change in our low velocity arena. One way to think of this is that the P is proportional to the number of molecules striking the surface per unit time. If they are going by the surface at a slower average forward velocity, they strike the surface more frequently and we see a higher pressure.

Bernie