I imagine I'd be confused too, if I was being questioned about the disappearance of my child - no mother would be thinking straight, and you'd be in a complete state of panic. I lost my son in Center Parks. He wasn't a toddler, he was 11, but I was beside myself with fear, imagining all sorts of things, and I wouldn't have known what 2 + 2 were if I'd been asked.
But the biggest point of all, to me, is that six years on, and the McCanns have been pushing and pushing for further investigation, never letting it be forgotten. It would have been so much easier to let it be forgotten and keep a low profile if they were guilty.