You'd think it's straightforward and additive. I added up all their listed instances for a couple different softwares. The math was wrong and overestimated the MAU count. I checked it a few times across a couple weeks. Couldn't quickly figure out a pattern to how the math was wrong each time.

Maybe they fixed it in the last ~6 months, but I wouldn't count on it. I believe it's the same Dev as pixelfed and loops. So, not sure they've done any work on fedidb in years (to my knowledge, they've focused on Loops and Pixelfed).

Edit: to clarify, adding the MAU of all their listed instances for a particular software was lower than the total MAU they reported for the software.

Edit 2: Just checked their Piefed listings and it's still wrong. Fedidb states: ~7.0k, actual addition: 6858 MAU.