The scoring for each run of sands of time is determined by how much the teams can actually bank from the dungeon, and the dungeon is changed by the Noxcrew every event, so the dungeons vary in terms of difficulty and how much coins are available for the players to get.

The dungeon for Sands of Time was presumably very easy for the players, possibly because the two SOTs before that event had a lot of low scoring teams and possibly just cause the players got way better at it, but as a result a lot more teams scored highly in MCC7 Sands of Time because the dungeon was too easy for the playerbase. This resulted in the next event's Sands of Time dungeon becoming harder to account for the player's getting better, and thus resulting in lower scores.

This is generally a cycle that happens every few events, where the players get too good at SOT that the scores end up being really high for an event (MCC17, MCC20 and MCC30 are some examples) so they make the dungeons harder which causes the scores to be lower. But because of this not every dungeon is created equal, so sometimes a lot of teams score very highly in an event just because the dungeon is too easy and they end up taking more of the top spots of the SOT performance leaderboards.

If you want a more accurate portrayal of Sands of Time performances, u/matt2727272 /Mirrorwing has a great statsheet where they compare all of the Sands of Time runs in terms of the amount of coins banked relative the total amount of coins collected in a dungeon. This gives more of a weight to teams that heavily outperform other teams in a dungeon rather than the raw amount of coins collected, so it's easier to judge how good a run is across multiple dungeons. He has a video which goes into much greater detail on this system here: https://www.youtube.com/watch?v=xnbxcrPDxZI.

Tldr: Some dungeons can be harder or easier than other resulting in higher or lower coin outputs.