If you
load up a world with default (not flat) terrain generation, and run
/kill @e[rm=1,c=20]/kill @e[rm=1,c=20]You would expect
20 things excluding yourself to be killed; the usual suspects: squid, bats, cows, pigs, sheep – and if you keep running the command: their drops.
What actually happens
That does happen, but, the chat reports some weird things being killed from time to time (if you do it enough), things I don't know how it's killing: item.tile.rail, item.tile.gravel
these screenshots were taken from -40 66 459, from seed 123.
What you are seeing is various items that have dropped throughout the world and can currently be picked up. When the chunk is loaded if there is a mineshaft near and water in the mineshaft it can pop the rail off, and various other conditions for the other items.