This is something that should not be overlooked.
A mate of mine is an amateur bodybuilder and in just about pushing 40. He's stronger than me in a few movements and is far stricter on his diet and gym routine, yet I have some ab definition and he doesn't - despite his grueling diet and gym routine. He's being trained by a guy who's had people compete and win so he's got the right people around him, right attitude, right determination etc - he's also lower BF than I am quite easily and while that shows in his face/arms etc, it doesn't show in his stomach.
You've got to think there's a genetic element in there. As a skinny kid I always had abs, easy, but as I got heavier and larger I've managed to maintain them with limited core/cardio compared to what some people say they do and don't have success.
N.B. Not saying my abs are special at all.