If you are like me, that last 1% is always much more work than it seems.
This. It's a wonderful, all-encompassing quote about why so many projects get abandoned.
There's, in my experience, two reasons the last 1% of the game content takes 10% of the game making work:
1) You often procrastinate, saving the most boring and most difficult bits 'til later, only working with them when you've got nothing else to do, or when completing them is a requirement to work with some more fun bit. But when the game is almost done, there's nothing more, nothing else, nothing
other to do than the boring bits you've saved for later because all the other stuff is complete.
2) When you have lots of work to do, you often try to do it quickly to make progress. As you've got less work left to complete on the game, one starts to work more carefully and wanting to make it perfect. (This is the reason you should never make the intro or the tutorial first and then make the levels in chronological order, by the way). If not careful, one'll set up impossibly ambitious goals for the last part of the game, which makes that last 1% of content really tedious to complete.