How are gift cards processed

I think I may have been " ripped off". I went to a restaurant. When it was time to pay I asked the server can I tip on the gift card or would she prefer cash. She said gift card was fine. It was $100 card never used. The tab was $73.00. ( rounding for easy math). When she returned it said I had a remaining balance of $11.00. I was going to leave $20 and after seeing that I couldn't. I called the gift card for transactions and the transaction was for $88.00. There was a $14 difference between the charge and amount do. So my question is ....on gift cards, are there holds or did she go ahead and tip herself? Thanks