Abs are postural muscles. Postural muscles are slow-twitch muscles. This means they can be worked more frequently with higher repetitions. One easy way to understand postural muscles is that they are muscles that are continuously being used. When you are walking, writing, breathing or resisting forces of gravity those postural muscles are working!
Try this while training your abs: Work on lower abs first. Then, your upper. Next, obliques and finally, your overall core. This type of training focuses on the weaker areas by doing the hardest exercises first. Your six-pack will thank you!