When I started taking pictures, 50% were keepers.
But as I grew into photography this number went down.
Currently, I'm happy with 3-4 nice frames in a roll.
I'm sure that as time passes, this number will continue to drop.
(I hope so
)
It is a simple equation:
The more pictures you take, the better you know what good pictures are.
The better you know what good pictures are, less pictures are considered by your self as "good ones", hence - less "keepers" on your rolls.
Add to this the fact that you want to keep on improving your photography and you get "progress".
I'm in progress, at the moment.