star wars - Why did R2-D2 have this item?

My apologies for the vague title, in the interests of avoiding spoilers.

In The Force Awakens, a large part of the plot revolves around

the part map that BB-8 had on a memory stick. The First Order have the rest of the map, and, as it turns out, so does R2-D2.

Is there any explanation as to why the latter has it, or how he obtained it? Or are there any upcoming comics or novelisations that may answer the question?

Also relevant: What made R2-D2 activate?

Answer

R2-D2 downloaded the map back in A New Hope, but didn't realise it at the time.

There's an article by Entertainment Weekly which recounts a post-screening Q&A with Abrams and co-writers Lawrence Kasdan and Michael Arndt. It explains how R2-D2 came to have part of the star map:

The story group’s thinking went back to the 1977 original movie, when R2-D2 accessed the Empire’s mainframe as the heroes searched for the captured Princess Leia. “We had the idea about R2 plugging into the information base of the Death Star, and that’s how he was able to get the full map and find where the Jedi temples are,” Arndt said.

Abrams says he chose to spell this out indirectly in the movie because he didn’t want the story to get bogged down in “how s–t happened 30 years ago.”

“But the idea was that in that scene where R2 plugged in, he downloaded the archives of the Empire, which was referenced by Kylo Ren,” Abrams said. Thirty-eight years later, in both our own and galactic time, that data becomes useful in The Force Awakens when a new droid approaches the dormant R2.

This doesn't explain how R2-D2's map has a missing piece – it seems unusual that the Empire's data would have such a specific hole – but apparently that's how it came to be.