Inconsistency of Difficulty Options in Client and Server

Summary:

This inconsistency means when you generate a singleplayer world, it will be set to Normal difficulty, but on the server file, by default, the difficulty is on Easy.

Steps to Reproduce:
1. Generate a world in singleplayer in any version of Minecraft, see the default difficulty.
2. Download the official server hosting file for any version of Minecraft, then see the server.properties file.
3. You will find the difference.