where the House wants to spend $15 billion and the Senate wants to spend $16 billion, so they compromise and end up spending $18 billion.

This kind of thing happens all the time in DC. Budget requests and appropriations are all a very expensive game. Requestors frequently won't bother to request something they know Congress will add in anyway, so they'll ask for other things. For example, the Navy might want a new destroyer but they know certain Congressmen in shipbuilding districts will put it in anyway, so they don't have to ask for it. They'll allocate the money that would've gone to the ship to something else, say aircraft, knowing the ship will be there when the allocation is finished.