The ECU (especially stock ones) can only compensate so much, especially when the vital sensors are narrowband. Combine that with common small internal wastegate flaps and modified parts, positive manifold pressure are highly likely to fluctuate from the factory preset boost. Often you will find OEM setups tapering off the psi when it reaches its peak boost setting due to lazy integrated wastegates on the turbine housing.

On my old setup, when winter would come by I would switch to the lower boost setting because psi would often go 1-2 psi higher than in warmer months.