Mobs spawn in light after changing difficulty

Mobs spawn in light level 8+ after changing difficulty. After a long frustrating attempt to mob proof my area I tested my methods on a new survival map. Light will prevent mobs from spawning on in a new world with difficult turned up. Once I turn difficultly down then back up mobs spawn directly in light. Extremely frustrating as I spent a very long time building a wall around my area and a torch grid that did not work!!!!