Did you also fork up the area where the plants were? Usually very few ever pull up with the plant. If you haven't done this, you can bet there are still more down there.

Unless you simply want new potatoes, you pulled them way, way too soon.
Had you waited till they died off, you probably would have had more potatoes and the "seed" would have been long gone.

I had to pull some of mine early due to volunteer squash wreak havoc but they had been in the ground since March.
Even a few of mine still had the mushy remnants of the seed plus the stolon where new potatoes would form were abundant.