Difference between Pick at something and Pick on something

pick at something—

1. keep pulling smth. or scratch it with one’s nails:

• He sat there idly ruminating and picking at the tablecloth.

2. (of food) take small, selected pieces now and again:

• He was just picking at his food, not eating it.

pick on something—select or choose smth.:

• We seem to have picked on just the right spot for a picnic.