Have you looked in the bedding under the feeder to see if they are just spilling it? That kind of feeder is pretty good, but some chickens still manage to throw the feed out with their beaks.
Can you easily see if the food level goes down during the night?
Or take the feeder into your house at...
Just what I was going to say!
Since that's about double what would be expected, the chickens are probably not eating it all themselves. Check to see if they are spilling a lot, or if some other creatures are eating it too (creatures that might eat chicken food: rats, mice, squirrels, deer...