that's it, thanks, LazyGun. That appears to be a significant change to the deal G4S originally signed up to. According to the BBC:
"G4S signed the contract with Locog in 2010 to supply 2,000 security staff to work at Olympic venues. In December 2011, the Games organisers asked the company to provide some 8,000 more, bringing the total to approximately 10,000 staff."
Locog might suddenly demand five times as many staff, but it doesn't mean the pool of suitable candidates is suddenly going to quintuple, and the time available for finding them is of course constantly shrinking.
I have no idea why nobody spotted the problems until last week. All the same, I suspect the focus on G4S may be letting Locog off the hook.