Why does the Olympic game cost usually run over its budget?

A study from Oxford university has found Brazil's Olympic game cost is 51% over its $1.6bn budget. Similar situations happened in most of the past Olympic games. Why has such overrunning budget repeatedly happened? Firstly, the organizing local government tends to provide an underestimated budget and an overestimated return from the investment in order to persuade the local support as well as the International Olympic Committee's support. Moreover, the limited transparency makes it easier for the organizing authority to provide these misleading guidances. Secondly, the 8 year time lag increases the difficulty in estimating the cost of organizing such an event. For example, at the time of Brazil applying to organizing the summer Olympic Games, the Brazilian economy was in a good shape; however,  the economy currently is in recession and has more difficulties to organize such a large event. Thirdly, the Olympic Games do not only need stadiums, it also requires many other supporting infrastructure, and some of these supporting necessities are not realized until they start to organize the event. In addition, cities have different situations and different Olympic games organizing plans; therefore, they cannot simply use the costs of other cities organizing their events for estimation. Therefore, the cost of organizing the Olympic Games overrunning the previous budget plan is almost inevitable.