Ulterior And Alterior both have the same meaning. They deal with motives and can be used in the same context. The difference between them is that ulterior is recognized as a formal word, while the alterior is not. Alterior is a word that came out of nowhere and appeared in television, film, and even literature.
It is unknown if an alterior will ever be accepted like an ulterior is. Many English experts feel that it will not because they believe the word was formed out of ignorance due to not knowing the correct spelling of the word. However, if someone uses the world alterior, one should know they mean ulterior.