Easy Street - re Olympics, the linked report states "The original budget was set in 2007 and was almost four times the estimated cost at the time London bid in 2005.
It was revised upwards after taking into account previously overlooked costs such as VAT, increased security costs, and an expanded brief to regenerate the lower Lea Valley area"
Not hard to be 'under budget' when you inflate the budget massively in order to achieve a political 'head above the parapet' goal....