Monsters spawn at light level of 9/10/11/12/13/14/15

All around my house is full of light but monsters spawn in a light level of 9,10,11,12,13,14 and 15.

Last time, spiders and creepers spawn at light level of 9 and higher. After I opened my door house, I saw spider waiting there. I don't know how did it get inside. My house is full of light level of 13. Lower, upper and very upper too has 13/14 light level. I don't even know what happened but all doors in my house are closed. 

 

 

how to reproduce:

 

Build a Huge house with 21 block horizontally and 15 blocks vertically

step 2:

Light up all the around the room

 

step 3: 

Try to load chunk by walking 32 blocks away from your house

 Go back to your huge house after walking 32 blocks away.

Then see what happened. Spiders, skeletons and Creeper spawn directly inside the house & breaking the rule of Monster spawning light level