Why USB 2.0, USB 3.0, PCI signals must be routed in 85E differential ohm impedance and not 100E impedance?
The standards define the electrical impedance that must be used to be compliant with the standard and to allow best interoperability between devices.
Only PCI-E defines 85 ohms.
USB is 90 ohms.
Many other interfaces do use 100 ohms.
85 ohms is used because it allows for less losses on longer traces and it can be more easily made with multi-layer PCBs with smaller height between layers that 100 ohms.