Hostile mobs spawn at high light levels.

My first day on minecraft, which is 1.7.9. A creeper suddenly spawned under bright daylight and started chasing me. Same thing happened in my creative world. Also one time I was watching a minecraft 1.7.9 video: https://www.youtube.com/watch?v=TjdKPDOvnfc
at 3:22 I saw a witch spawn under sunlight, with another witch nearby already spawned. Then a gold-armored skeleton spawned under same environment at about 3:39.