You just have to think about what that chart's telling you.
In some atmospheric conditions, the engine can produce more grunt than it is 'allowed to' by the designers.
This is a common thing - depending on helicopter and engine type, there may be a limitation on the engine itself (eg don't pull more than a certain manifold pressure under certain conditions or damage may be done to the engine by internal pressures greater than it was designed for), or a stated maximum torque allowed to be applied to the drive train.
This chart says "In the 'non-full-throttle' zone, don't pull any more than this much MP or you may damage the engine. In the 'full-throttle' zone, your engine is producing as much power as it possibly can anyway, so you don't have that problem."
To clarify, looking at the R22 Beta 5 minute power chart (O-320-B2C), there are a bunch of different alt/temp combinations that would give you a MAP limit of 24 inches - eg Sea level & 0 C, 1500 ft & +10 C, 3000 ft & +20 C.
This tells you that if you're flying at any of these, don't pull more than 24 inches or you may hurt the engine. Note that for a given height, as it gets warmer, you're allowed more MAP.
However, following the 24 inches line across, when you get to about +25 C and 4200 ft, you reach the full throttle line. If you were flying under these conditions and raised the collective from some low power setting, you could expect the MAP to keep increasing and rotor rpm to be maintained until you got to 24 inches, where the throttle butterfly would be fully open. Raising the collective any more would cause the rotor rpm to droop, because the engine would already be giving you as much as it could.
I've cut a short story long there, but basically, to the left of the full throttle line, you have to physically stop yourself pulling more MAP than you're allowed. To the right of the line, you won't be able to reach the MAP indicated at the intersection of your temp and altitude - the engine won't do it. Yes, you can fly there, but the maximum collective you can pull will be limited by engine capability rather than your adherence to the limit MAP chart.