A pressure calibration can be impacted by the environment around it.
Temperature - Almost everything that measures pressure is also impacted by temperature. Pressure calibrator manufactures will characterize and compensate,but they still perform best over a given temperature range.
Gravity - Acceleration of gravity varies by location.A traditional deadweight tester is impacted by this.
Head height - A large difference in elevation between the reference and the device under test will create an error.