I'm having a constant problem with Drifters spawning in my cellar. My cellar is 7x7 with oil lamps in each corner and another in the middle. I even put three tables in the middle and made the floor from slabs (the upper half cause I heard that blocks spawns). Seems to be worst when it rains. That said, just had 2 spawn in my workshop which is 8x5 and lit with a lantern in the middle.
I attached a picture. It's symmetrical so everything you see is mirrored on the both sides except the door in.
Clearly I'm not understanding what the actual spawn rules are, because I'm dead certain I'm keeping above a light level of 7 in all areas.
Any help of advice is appreciated