This was answered in an issue of Airliners about 10 years ago and I thought I remembered the 255k weight limit and the detail on the 75's.

So, if the 75 triggers wake turbulence warnings, why then would it not be referred to as heavy, despite the weight issue? As a pilot following, I could care less about the actual weight of the plane in front of me unless it had the potential to cause me problems- the 75 should just be included in the heavy category for practical purposes