If one pulled more collective, MAP is supposed to stay at that full throttle value, but RRPM droops due to higher angle of attack/drag.
Some of us however report here that this is not necessarily the case.
'The book' actually says very little to explain the MAP placard, but what's important to note is that at low density altitudes the engine is capable of producing more torque than the drive train is rated for so you can easily pull more than the MAP chart says but in doing so you are over torquing the gearbox.
As density altitude rises the engine cannot deliver the same power and you reach a point where the MAP chart is giving you a theoretical maximum for full throttle, which may be less than the torque the gearbox is rated for, and may not actually be achieveable if for some reason the engine is underperforming. Keep pulling collective here past the MAP placarded value and the RPM will droop. You see this at approximately 6-7000ft altitude (ISA temps) in a Raven II. You'll also feel the governor making large throttle adjustments on the twist grip as it starts to operate near full throttle, and if you lower the lever too quickly from a high MAP the RPM may get high as the governor is slow to close the throttle.
However, back to the first issue which you started this thread with. If the are seeing values much above 15" at 102% with the collective fully down anywhere near sea level I'd suggest it may well be the line to the MAP gauge leaking, or a failed MAG (be this would easily be identified in the mag checks).