Where is my mistake in extracting percentages for Urban Atlas 2018 in QGIS?

When extracting percentages for Urban atlas and adding up all the percentages for one address I either end up at ~60% (80 percent of the times) and also sometimes at 120% (the remaining 20%). Do you know what I am doing wrong? Here is what I did:

I have the x/y coordinates of addresses in GB, Germany and Ireland and downloaded the 2018 versions of Urban Atlas (all cities for Ireland, Germany and GB). I included the addresses by the step "add delimited text layer" and made buffers with the radius of 200m, 500m, 1km and 2km (using MMQGIS). I merged all Urban atlas pieces to one big map. Then I reprojected both layers to EPSG:3035. I made a new variable in the buffer shapefiles called "catchArea" that included the $area of the current buffer. Then I intersected the buffer-shapefile with the all-in-one UA-map. After that step I made a new variable in the intersected file with $area (called classArea) and then made another variable with the formula: (classArea/catchArea)*100.

Now adding up all the percentages I don’t end up at 100% as it should be. This error does not exist if I have a buffer with a radius of 10m but I do not get less percentages with the 2km buffer. (So it is 60% in 200m and 60% in 2km most of the times).

I now started to make an extra step with dissolving the all-in-one map first and now intersect it. Dissolving worked fine but intersection now runs for 5 days now and there is no end in sight. Even if this is my solution it takes too long to be a solution and I am hoping for your swarm intelligence to tell me where I have made a mistake.