Yes, I figure this was due to a bit of retconning on Lucas’ part. When he wrote Star Wars: A New Hope, perhaps he intended “Darth” as a name rather than a title. It wasn’t until the Phantom Menace he introduced the usage of Darth as a title.

Even so, how many times are other character addressed by their titles, such as Padawan, Master, Senator, etc., not to mention military titles like captain and admiral. In that context, I think it could make sense for Obi-wan to call Vader “Darth.”