That's for fifth wheels, of course. For pull-behinds, the length is from the center of the ball to the back of the trailer. If you think about it, both lengths represent the trailing part of the unit from the point of connection to the tow vehicle. At least for the purposes of "will my rig fit this site", bumper pulls have the advantage of being closer to actual length.
Also, someone mentioned on the forum recently that they heard that the first two digits in the four-digit model numbers used for units such as the Big Country and Bighorn represents the length from the front of the main frame (where the frame transitions upwards to the upper deck) to the rear, in rounded number of feet (roughly 4 feet less than the pin to rear of unit length). That seems correct given we've had two different 36xx model Big Country units that were both around 40' pin to rear.