I usually notice that surfaces are not exact copies or exact mirrored copies by looking at their isocurves.
The right side is the mirrored side, and as you can see on the picture it's topology is slightly different.
It'd propably be better to avoid mirror feature and use copy and paste special transformation instead, or maybe FMX mirror would do the right job. Not sure.
One question, how does the Flip Normal feature work in Creo Parametric? I can't seem to be able to edit it's definition.