It's called a "bait and switch". I've heard of it happening when people buy a puppy from a picture on an internet website. One member on another forum thought she was getting a 10 week old female and was shocked to find a male who was about five months old in the crate when she picked the puppy up at the airport!
The fact that you are questioning this breeder makes me wonder of you are completely confident about getting a puppy from her. If you have any doubts, trust your instincts and find another breeder.