Mobs spawning on surface during daylight

Created an buffet world, overworld desert. After playing for several game-hours over a couple of days, opened to LAN and went exploring with wife. We approached a town in a flat area and saw 3 creepers, a skeleton (which did not burn until it moved out of the area) and 2 spiders spawn in a small area. On the other side of that town as I climbed a hill, I saw 3 zombies appear and start burning as they approached. Light levels were 15 in both cases. First occurrence was midday, second was shortly after noon. May be very difficult to duplicate as it happened once while searching about a dozen towns and temples.