I prefer the term "stock".
WotC sells a stock car. All WotC parts, all WotC design. They also sell official parts, upgrades, and accessories made explicitly to be compatible with thier stock. However, their is no reason why you can't take your stock car and fill it with custom and aftermarket parts. You can repaint it, replace the tires, even replace the engine if your brave enough.
I think the issue is that people use the term "D&D" to refer to a lot of things: out of the box stock, lightly modded, customized aftermarket, heavily rebuilt, and even other similar products (ie Pathfinder). Further, people disagree what stock should even look like; some people want a fully usable car requiring no modifications and others want a box of parts they can make a car or a bicycle or whatever out of it.