rectus abdominis is innervated by the lower 6 or 7 thoracic nerve-root segments. So since it has multiple nerves, it makes physiologic sense that regions of the muscle could be more or less active than others. If T6 is firing, the more superior fibers of rectus will contract. If T12 is not firing at the same time, the inferior fibers will not be contracting. Think of rectus more like 6 or 7 muscles connected in series, any of which could (at least theoretically) contract on it's own. In reality, we don't have fine enough control to contract just 1 segment, but I don't have any trouble believing that the upper segments could be more active at a particular moment than the lower, or vice versa.
The question for me is: why are they separately innervated, and what about certain movements would stimulate one part over the other and vice versa
Not to mention if one is contracting harder, wouldnèt that stretch the other moreÉ