The "Pretty Guardian" title has been in use ever since the live-action series dropped waaaay back in 2003, although to my understanding, even the faithful Viz redub of the series drops that part from the title. That said, a lot of fans just call them "senshi" because the history of the term is, um, complicated (another valid translation of senshi is "Soldier", and in fact if you listen to the BGM during the transformations, that's just what the singers are saying), and they want a common term everyone can agree on, something that can't possibly be inaccurate.