Boss tablet can't add bosses to some areas

I have found irridated and corrupted area next to tower, so I thought it will be a good target for a boss tablet to gain next passive point. I have cleaned up some areas around to ensure tablet will target specific area. I've tried to use boss tablet which adds 7 bosses to nearby areas and got warning that there are only 3 places left. Which was little bit surprising. So I've used tablet which adds 3 bosses only and of course it didn't target area I wanted. So, after 3 spots were influenced then there shouldn't be any valid areas left. But there are at least 7 which still technically should be able to contain a boos. I've marked tower and those areas which were in range of the tower:



Why those areas cannot be influenced by boss tablets?