Thats ok, because the mattress motor doesn't need to run that long, its never run the batteries down, I usually misplace them by next year or stick them in something else.
I've noticed a lot of store bought rechargeable D batteries are the same capacity as AA also, to get the full capacity you have to order them from a reputable brand, i.e. no energizer, duracell, etc that are trying to hit a walmart price point.