According to f's notes on part one of the JKR podcast

-- Hannah Abbott is not Muggleborn; she's pure-blood. And then Jo realised that she forgot that she was meant to be Muggle-born (as indicated in her notes from years back). So on Pottercast she decided to "split the difference and call her half-blood." --

Barring the randomness of just deciding things IN an interview, I feel pretty darn smug about the backstory I developed for Hannah now, because it completely fits this.