Why do governments underspend on their budgets?

It was pointed out to me recently that the City of Toronto regularly underspends on its capital budget. It defies logic that, after all the teeth-pulling and hair-tearing the City goes through to pass a budget each year, with all the recriminations and angsty calls on other levels of government to better fund Canada's biggest and most needy municipality, it then doesn't even spend it all!

Now, I know it's not quite as simple as that, and the interplay between the operating and capital budgets, and between all the various elements of the capital budget (some of which are allocated to specific areas of spending) can be quite complex. 

But the bottom line seems to be that the City, which spends much of its time complaining that it is underfunded, does not spend all the money it is allowed to spend on the upkeep and maintenance of the city's buildings and infrastructure. And not just once, but EVERY YEAR! Particularly under John Tory.

Then, I read just today that the province of Ontario does not spend all of its allocated budget. $6.4 billion in the first three quarters of this year alone. And again, not just once, but EVERY YEAR. Unspent money has been building up in reserves year on year, because ... well, I don't know because. I understand that governments can't spend MORE than their allocation, but they could at least make some attenpt to spend UPTO it, especially given that so many programs innovations many areas - health, housing, transit, etc -  are chronically underfunded.

What's with all the underspending by conservative governments