It's the safest move for NBC at this point since the show is on the fence. It isn't doing well enough to commit to it, but it isn't doing so bad (by NBC standards - especially on a Friday night) to cancel it. The inconsistent ratings and large live+3day bump means the show still has the potential to grow, but there's no sense in ordering the back 9 in case it doesn't. NBC will likely air all of the episodes regardless, so this buys them more time than just 4 episodes to better judge if the show is worth keeping. This doesn't bode well for the show, but the fact that they haven't outright cancelled it makes me think NBC is playing the waiting game to see just where the show lands. This also prevents the producers from shopping the show to other networks in the meanwhile.
The question I have is what NBC is planning to air once Constantine's 13 episodes are finished. Hannibal is not likely to return until April or May, which leaves the timeslot open from February to April.