There are two things to consider when a 'Gubmint' department is slow to pay.
1) The admin in the department is tardy and as a result documents are not submitted timeously.
2) The relavant department has blown its budget and has to wait for further financing, this is the worst as usually it means waiting for the next financial year.

But of course I agree, they demand immediate payment, and 'Woe Betide You' if for some reason you are tardy.
They will slap a judgement on you which is like saying, 'We want that fruit from the top of the tree, but we need to tie your hands behind your back before you climb up'