That's because the muscles that actually give you a flatter stomach are the transversus abdominus and lumbar multifidus. These internal muscles play the largest role in creating a flat stomach.
I personally don't like the look of a six pack on a girl, but a flat tummy is a must. And, the key to this is to exercise those internal muscles that I talked about earlier. It's called the vacuum exercise.
How do you do this? Well, it's REALLY simple.
1. Exhale all the air that you can.
2. Pull in your stomach as much as you can.
3. Hold for 10-20 seconds.
4. Repeat 3-5 times.