This is explained by the fact that there are two different logics that clash when creating a film poster.
The short version is that the names are listed from left to right in order of importance, while the photo of the most important actor is preferably placed in the center.
We understand that with this logic, the star of the film cannot be both left and center. Hence the frequent inconsistencies observed on posters.
The long version is that the order of the actors’ names on a movie poster is the same as in the opening credits of the film.
We start with the most important actor (bankable), then come the following, in descending order of importance.
The position of the name is subject to separate negotiation.
For the general public, it’s a detail, but in Hollywood, we fight tooth and nail for top billing , that is to say being at the head of the distribution.
We can cite the famous example of the Infernal Tower , where Paul Newman had top billing before Steve McQueen demanded the same privilege.
To get out of this mess, the studio designed a poster where the names of Newman and McQueen were each in the first position: one reading from the top, the other reading from the left.
Thanks to this trick, the two stars validated the poster without making a fuss…
Regarding the graphic design of the poster, we therefore try to put the star in the middle and in the main, but the poster must first be successful from a marketing and/or artistic point of view. We will therefore favor these points rather than systematically placing the star in the middle and roughly.