I am simply speculating, since I have no idea what the dyno room/sensor setup is.
Man pressure could mean Manometer pressure, ie the air pressure in the room. But why was it dropping so much with engine rpm?
Really questions for the dyno operator/shop.

14.7 psi = 29.92 "Hg
12.1 psi = 24.6 "Hg

So both columns seem to be measuring the same thing.
If at low rpm (air flow), the Manifold was at 30.1 "Hg and at high flow 24.6 "Hg, is that a reasonable pressure drop across an 1150 Dominator carb at WOT?
(30.1-24.6) = 5.5 "Hg