It's most noticeable when you switch between difficulties. When i switch from any difficulty that allows monsters to spawn, the mob cap still remains higher than zero. On the screenshot it says M:27 and Local Difficulty: 0.00 - peaceful.

Because of this mob farms don't work at full capacity.

The way i recreated it is like this:

1. Started a new world.
2. Travelled about 350 blocks in one direction with Easy difficulty.
3. Switched to Peaceful difficulty.
4. Boom

I mostly play on lower rendering distances - 6, 4. Always either 8 or lower than 8.