I used to work in a library and had access to newspapers from all over the USA. I collected articles on various subjects and was amazed at how different newspapers publish the same AP story. Based on which AP paragraphs were printed in a specific newspaper, you could easily identify the left or right bias of the particular newspaper. Headline wording accompanying AP stories seems to also be up to the individual newspaper. By printing only part of the AP story, the newspaper has the ability to modify the slant.
I'd bet the ranch that the "left bias" newspapers greatly outnumbered the "right bias" ones.
AP itself is a consortium of newspaper organizations which contribute to a common operation in order to reduce the reporting costs for each member paper. No wonder why the AP stories are left biased: just about all the member newspapers which support it are!