Yeah - I'm sure there's some of that as well. I think its mostly just 'design by committee' for the most part though. A lot of companies don't do sponsorships, etc. so they don't really need to worry about it.
As others have mentioned, it can also be tailoring a position for a specific candidate just so they have a reason to hire the person they already selected.
Other times I think someone left so they post the skills that the previous guy had.