Because muscle (mesoderm) and peripheral nerves (ectoderm) develop from different embryonic germ layers, the pattern of innervation is somewhat variable. The muscles are condensing from mesenchymal tissues more or less in place, and nerves are growing out into the muscle masses to innervate them. Because of this, the specific spinal segments that innervate muscles can vary somewhat. This is particularly true in the brachial and lumbosacral plexuses. In the brachial plexus for example:
Grayed out text for C5 in the median nerve and C7 in the ulnar nerve suggest variable contributions from those segments.
In this case, rectus abdominis is "always" innervated by at least T7 to T12. But sometimes T6 as well in some, but not all, people.