another way to look at it...
due to gravity, all objects fall in a vacuum at a constant rate or speed of 32 feet per second squared...
Because they lack wings, all bullets start falling the moment they leave the barrel (assuming your rifle is level when fired). It takes a split second to reach 100 yards, but even then, gravity has pulled your bullet down a bit in that short time span...
Because of air resistance, the bullet starts to slow down as soon as it leaves the barrel. the farther it goes, the more it slows down. Eventually it would actually stop and drop if it didn't hit the ground first...
it takes longer for it to go from 100 to 200 then it did 0 to 100. In that slightly longer bit of time, gravity pulls it down a bit more than the first 100 yards. It keeps getting slower and slower, allowing more TIME for gravity to pull it down further while it's traveling the same DISTANCE to the next target. Eventually it just hits the ground.