In most actions with a Purdey bolt, the front of the bolt rides on the portion of the front lump that is below the bite when you take your thumb off of the opening lever. The larger angle of rotation around the hinge pin demanded by an O/U that is underbolted means that the area below the bite must be greater. If that extention were not there, the bolt would snap in under the lump when the gun was in the open position, and you could not close it without pressing the opening lever again.
All the best, Mal