McMac

Google Books gives 254 hits for "Declan McManus" as a phrase, and 138 for "Declan MacManus". The more definitive-looking books have "MacManus", e.g. Elvis Costello: a bio-bibliography and Complicated Shadows: The Life and Music of Elvis Costello. But then again, The Rolling Stone Illustrated History of Rock & Roll, The Billboard Book of Top 40 Hits, and The Houghton Mifflin Dictionary of Biography all have "McManus"; and some books have both (Complicated Shadows, for that matter, gives his father's birth name as "Ronald Patrick Ross McManus").

Is there a genuine apathy for "Mc-" vs. "Mac-", so that either one is actually correct? Is it the case that even hopefully reliable sources just don't care enough to get it right?