"Do you want change" - Tipping?

When a waitress brought me a check for $13.xx, I placed a $20 bill on top of the little tablet. When she came back, she asked me if I wanted change. Frankly, I was shocked.

1. Did she really expect a 65% tip?
2. Given the small size of the bill, do people normally round up?

BTW - when I told her I wanted change, she brought back a single and a five. Of course, it made leaving a normal tip 15-20% a hassle.