Monsters spawning during day

When i Optimize my survival world to 18w33a, after night, i saw a slime wich spaned in Extrime Hils next to my house. I killed that slime, but then, a skeleton was shootimg me. After that, the crezeper explodet, but I didin't take any damage. Then, I saw a lot of monsters. I changed to Creative mode and I also saw that bats were spawning next to the trees. My question: why? (oh, and by that i coudn't build a mob grinder, that was too anoying for me)