Yes, I think that for a transfer to be free(for the recipient) it needs to be within SEPA and EUR. It says here that:
"With SWIFT transfers there are a few things to bear in mind:
When your money is in transit, it may be processed by an intermediary bank. They may also deduct a handling fee.
Depending on which fee option (SHA, BEN, OUR, REM) you choose while doing a SWIFT transfer to Revolut, intermediary, originating bank and beneficiary bank (namely Lloyds) can apply a handling fee for that transfer."
Maybe that’s why you lost 6 CHF.