Creeper Spawned in Well Lit House

A creeper spawned in my house when I came upstairs and exploded. I have not had mobs in my house for 2 years because it is so well lit (other than endermen). I have walked every inch of my house at least a dozen times and there are only a couple places with 8's but every where else the light levels are higher and I have done this once with smooth lighting off. The kitchen where the creeper exploded (the spot is marked with a trap door in attached photo) is lined with windows, has sea lanterns in the ceiling, glowstone under the floor along the wall, has a hanging lantern and torches on the table nearby and by the windows. I cannot see how this could have happened unless it is a bug. Please advise.