Someone previously mentioned the roof being in darkness. Pity I can't find any previous shots of the Clutha at night, but if he was expecting to land on what he thought was a flat bit of ground 10ft lower down, the flare probably came a bit late (if at all) before the hard impact with the roof.
It's the Sun editor's version of falling like a stone with rotors not turning that's got me, and he seems to be the only witness.