The spending figures mean something, the deficit figures are meaningless because whatever happens the price of oil won't be $40 a barrel - it might average $30 and give a huge deficit or it might average $50 and create a huge surplus (as has happened repeatedly in recent years) or it might be anything at all, really. The $40 is just a best-guess assumption, and historically the assumptions have usually (and no doubt deliberately) been very much on the conservative side.

It is very important for State spending to stay up throughout this slump in order to ensure there is a strong flow of money in the country. If Qatar started to pull back from its "vision" programme it would undermine the economy and probably trigger a downward spiral of job losses, project delays etc. (and if you see these already, imagine how much worse it could get). State spending is the thing that insulates Qatar from much of the fallout of the global slump.