Whenever changing the simulation distance of an existing world or creating a new one, the true simulation distance remains at 4 chunks. The slider displays what was selected as the simulation distance but the true simulation distance remains at 4 chunks. I discovered this when i was testing distances for an automatic farm i was building. I then went into a superflat and used command blocks to display the hostile mob spawning positions and no matter what simulation distance was chosen, the true simulation was that of a 4 chunk simulation distance.