They prefer not to take additional nodes if not needed due to risks. I had 3 taken, in the breast and 1 node was actually 2 when looked at. I have same pathology. All very treatable. When they do the additional clearance they take a clump...Apparently the nodes are all caught up in fat under the arm so it's very hard for them to say how many will be taken. If after clearance it's under 3 you might not require radio. But they might do it regardless to be sure.
Hang in there....know they were doing the best for you to reduce risks of lymphodema.
This link really explains it and the pictures are very helpful I think.
http://beasurvivoravera.com/axillary-lymph-node-dissection/Kath x