Apparently the booster signaled it didn't have enough fuel to risk it. Either because it consumed more then expected on the way up, or a measurement error potentially.
That'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.
Amazing that a one-second difference can matter, but those things go through tons of fuel every one of those seconds!
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.
Amazing that a one-second difference can matter, but those things go through tons of fuel every one of those seconds!