Zombies have spawned in a well lit building

A zombie got a few of my villagers a while back and I figured I must have been careless with a door somewhere, but then one spawned today inside a building that I had checked for lighting - it's lit up to block lighting level of 9 or better everywhere. I was in the room directly below this, so I killed it and immediately came up and checked the lighting and everything - no zombies should have spawned here.