they are all even. Level at the top.
He did this crap to avoid making any cuts on the bottom so they could stand up straight. Why? Dunno. Lazy? Couldn't figure out how to do it easily?
Myself, I would have put the first one up. Straight (duh!). Then held the second one up along side it. Upside down. Take a measurement on the bottom of the picket (which would be the top for the moment) Draw a line across it using a carpenter's square and pencil. Then make the cut, and nail it up, making sure it was plumb and true.
So I think to save some time, he held the level across the top and nailed the pickets up accordingly. It's WRONG, but there you are.
Seriously, fence contractors could use this as an example of how NOT to build a fence. Moron!