The - is to create an element of randomness to the game.
So a PA of -10 with generally have a high PA, from about 170-200.
-9 will have a PA of 150-180 and so on...
I'm not sure if they are so definate as that, but you get the idea.
But of course if you are creating a star player then just give them a fixed PA (190-200) and then you'll know they are as good as you want them to be!
And no, there is no way of having all attributes at 20. If you set them all at 20 they reset to 16 in game. You can however sacrifice some attributes and this will allow the important ones to be higher.
There are some much more detailed threads on these two topics if you have a search