This needs better communication, but you will never “lower your rewards” by doing the world events.
Imagine there is a list of “the top 8 things you did”, which is what the delves use to calculate.
Level 8
Level 8 - this is the first level of reward. Since of the first two, both are level 8, you get a level 8 reward
Level 8
Level 8 - this is the second level of reward. Since those two are also level 8, you get level 8
Level 8
Level 1
Level 1
Level 1 - this is the third tier of reward. As you can see, it has an 8 on top and 3 level 1 at the bottom.
Now if you were to do three level 7 delves, the list would look like this:
Level 8
Level 8 - reward still 8
Level 8
Level 8 - reward still 8
Level 8
Level 7
Level 7
Level 7 - reward is now level 7
Level 1 - Pushed off the list
Level 1 - Pushed off the list
Level 1 - Pushed off the list
And if you then did three MORE level 8, the entire “top 8” would be filled with level 8 delves and all three rewards would be level 8.
If you have done 8 level 8 delves and your third tier is not a level 8 reward, then some of those delves either weren’t level 8 or didn’t count as level 8, maybe because someone you played with lowered the tier level and nobody noticed.
You have no way of knowing what those were, though, because unlike the mythic system, you have absolutely no way of tracking your actual delves that I (or the game) know of in the same way that you can track mythic dungeons.
“Unless you have data that proves us wrong, you gotta assume we’re right”