My guess is most 3rd party applications set their world scale to cm by default.
Why cm?
When dealing with content development cm is easier to deal with as it directly converts to units as a ten base value.
For example I have 3ds Max and Motionbuilder setup to use cm for world scale so applicaion A+B=UE 1unit=1cm.
When working with fidelity of asset it’s best practice to work with values expected by the target application.
Either way it’s best to work with a known world scale than not having any, as in UE3 