I dont know that there is any ultimate source and whats most important is the buyer and how they will use the property.
When entering MLS data most realtors are trying to create a picture of what the house is and can be. For example, there is a floorplan in my neighborhood which often has 3 BR but has 3 separate places you could create additional bedrooms within the existing structure of the house (i.e. not expanding the size beyond the current footprint). If you list it as a 3BR many buyer swill pass over it even though it is easily made into a 4, 5 or even 6 bedroom. So sometimes agents list the bedroom count higher so buyers will come see for themselves and make their own decisions whether the house works for them or not.
FWIW, I consider a real BR to have 4 walls, a door, a window and a closet.
