At the last moment the exhaust turned green. That's consistent with the fuel running out, the engines running oxygen rich, and subsequently "engine rich". So I would put my money on actually running out vs a measurement error.
replyI heard somewhere the engine efficiency is still somewhat variable so they burned more fuel than anticipated.
replyThat's basically what Musk said, yes: that they need to work on both engine performance and performance consistency. He said they'd be cutting off the main engines one second sooner to provide more margin for boost-back.
replyAmazing that a one-second difference can matter, but those things go through tons of fuel every one of those seconds!