Why did this designer decouple to PE-ground?
This was probably to meet RF EMI emission standards from the equipment back out of the mains lead. This prevents RF on the transformer with respect to case ground propagating along the mains lead with the supply cores and the PE core acting as a transmission line.

A more common way to improve this spec (likely as well as using C2,3) would be to connect the screen winding of the transformer (the vertical line between primary and secondary) to PE ground, to intercept most of the RF picked up by the power supply.