My impression was that Lucas had this big story (draft), he took what he thought was the core (most interesting) parts of the material, and started from there, initially, ANH never had the episode 4 in the crawl, after SW started becoming a hit, and since some plot and character issues were skipped over, he thought it opened the possibility of doing more sequels and possibly going back and doing some of the backstory, so leaving room for future possibilities, and adding an interesting touch he placed the label of Episode 4 on ANH and made the bulk of the material into what he thought would be the middle three movies which is the OT. I read the Journal of Whills awhile back, The "backstory" and what we see in the PT is extremely scarce and scattered throught the story, I see some character names / Planets and some familiar ideas, but nothing that really resembles the PT even as far back as 73.

For those who haven't seen early drafts before ANH look here:

Early SW Scripts