Thanks Scrap and Adam,

I looked up the muscle types you were talking about and found this:

Muscle Fiber Types and Training

I guess reading that it sounds like maybe your abs tend to be made up of a higher proportion of slow twitch muscle fibres since they have a role in maintaining posture etc and that means they're likely to be more resistant to fatigue so you can work them harder/ more often maybe.

You were right on the money there Adam