I had exactly the same issue as yourself with my Sette 270W. I have 2 shims installed (supplied with the grinder) underneath the burr cone because otherwise I could not get a fine enough grind for espresso. In my case, when I disassembled the burr I found some grounds between the shim and the under side of the burr cone on one side. See attached photo. This was enough to offset burrs so that the grind coming from one side was a smidgen larger than the other. I cleaned the shim and burr cone and now all is good.
I'm surprised that cleaning the slots fixed the issue in your case. After all, as long as the grind size is consistent, then even if it comes out of the grinder more on one side, it is still consistent in the portafilter after you have tamped...
Cheers.

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