Why are mobs spawning at light level 7 in my house? (Minecraft v. 1.12.1)

I built a sky mansion, but sometimes mobs spawn inside. Light levels are all above 7 where there are like three or four of them. All other fields are up to 13 light.

It happens very rarely, maybe once every 10 days, but if it's a creeper, once is too much.

Any ideas as to why this is happening?

If the block that they're spawning at is next to a wall, light up the other side of the wall. Grouped spawning mobs only need one of the blocks in the spawn area to be less than 7 to spawn and will sometimes spawn on other sides of walls.

If they are not spawning next to a wall, or the opposite side of the wall is solid and not hollow, check the lighting again - they can spawn at light level 7, but no higher.