No idea, each force is different and buy guns and ammo at their own whim and change them frequently depending which way the wind blows.
I expect they shot him twice to start with, assumed he was dead, retired to a safe'ish distance to await EOD, whilst high fiving and organising tea and medals. I mean, who wouldn't have thought he was dead.
They probably missed all the vital organs, he came round, they thought 'buggar' , as you would when you think a PBIED is about to be clacked off. Got off as many rounds as possible to neutralise the threat, and some of which missed, as you would expect.
Hindsight is a wonderful thing and the 9 o'clock jury (as we used to call them) can disect it slow time, with lots of 'what ifs' and 'should have's'. Bottom line is, they had a fraction of a second to sort it out, and fortunately nobody else got hit.
I expect the hit rate in close quarter fighting is much lower than that day, so the fact a few rounds went astray is more than forgivable.
I have to say that Narwhale Tusk man and Fire Extinguisher man were the real heroes that day! Together with the Polish bloke with the lance in Fishmonger's. If it hadn't been for them, a lot more people would have been injured or killed.