A geometrical difference between target and actual geometry is called warpage.
In the plastic industry this is caused by the developing stresses during the process.
Stresses arise due to the different shrinkage degrees in the part.
Varying Cooling rates, Orientation of the molecules, in-mold constraints (limitations for the shrinkage), uneven wall thickness/packing pressure are responsible for the differences.
These effects can cancel each other out or enhance one another.