Do you have a source you can reference? The Wiki reference to the topic is poorly written and confusing. Gray was the more senior of the two, he was already a physician at the start of the project while Carter was not yet, and by all accounts I have read Gray was the originator of the book idea.
The Wiki post is confusing, and seemingly contradictory. Carter was away in India at this point, so I assume the intention of the entry is to say that Gray took advantage of his absence to minimize his partner’s title credits?
“The first version of Gray's Anatomy, published by Parker, presented Gray and Carter in equal-sized characters names, followed by a caption on a smaller scale with each's titles. Gray was strongly opposed, believing that it took advantage of the absence of his colleague to claim that the name of his illustrator was written in smaller letters with no titles that Carter had obtained.”