Your FIXED calc is not taking into account all the filters on the sheet. That's part of the design of a FIXED calc.
Tableau has a very strict "order of operation". (Google for this to get details about that.)
If you want the filters to impact the FIXED LOD, right click on each filter and choose "add to context". This will push the filtering operation BEFORE the computation of the FIXED LOD.
Thank you for your help.