Have I missed something here? What is this urban myth?
Surely:
The induced flow hits the ground and spreads out in all directions essentially forming a divergant duct (reduced velocity = increased pressure) therfore increasing relative pressure below the disk.
An example of this can be witnessed on many helicopters (dependant on the position of the static vents I think)when you pull in power to take off and the VSI indicates a rate of descent (relative pressure increasing)