There are several reasons for this:
1. It varies from writer to writer; one writer may depict a character as Tier 10-B, while another portrays them as Tier 4-B. This can be attributed to plot convenience.
2. Power levels differ across planets, but writer inconsistencies play a role again. Sometimes, a Force user on a planet is shown as extremely powerful, while other times as very weak.
3. Force users generally have low piercing durability, meaning they can take significant damage from piercing weapons with relative ease.
More reasons could be listed, but these are the ones that come to mind right now. Overall, it's mostly due to writer errors.I'm not sure if this would make a significant difference, but I wanted to point it out. There’s one undeniable truth in Star Wars: no ordinary human can match a Force user. So, it ultimately comes down to writer inconsistencies.