Primarily because the absolute pressure controller senses the
temperature of the induction air between the turbo compressor outlet
and the fuel servo and adjusts the "absolute" maximum MAP according to
temperature by controlling exhaust wastegate position.

When setting up this controller, a temp probe is inserted into the
induction airstream and the temperature at max power measured. Using a
Lycoming chart, the max MAP is adjusted according to the indicated
temperature.

MAP/RPM is a relatively crude way of indicating horsepower. In simplest
terms, when the induction temp goes up, horsepower goes
down-consequently when induction temp goes down, horsepower goes up.
The absolute controller, in theory, maintains this temp change/HP
curve. But an initial start/rig point must be established after engine
installation or absolute controller change.

TC