Bug spawn slime & bat in server

When I start a server with this properties :
#Minecraft server properties
#Wed Nov 21 21:11:42 CET 2012
generator-settings=
allow-nether=true
level-name=Speed Quizz
enable-query=false
allow-flight=false
server-port=25565
level-type=FLAT
enable-rcon=false
level-seed=-1501881007
server-ip=localhost
max-build-height=256
spawn-npcs=false
white-list=true
spawn-animals=false
snooper-enabled=true
hardcore=true
texture-pack=
online-mode=false
pvp=false
difficulty=0
gamemode=2
max-players=6
spawn-monsters=false
generate-structures=false
view-distance=10
motd=...

I have the bad surprise to see a lots of slimes and bats on the ground.
Any idea why ?
(There is a lots of bedrock's particules too)