Stop. Take a step back.

Airlines have always liked marketable prices for their fares i.e. 39.99, or 27.99, etc.

But they like to do this in local currency at both ends of their routes, and they may have different currencies.

Hence, in traditional airlines, they file fares 'exABC' where ABC is a location, with fares in local currency.

For passengers combining flights, out and back, this is usually done on a half round trip basis - but regardless, it is done using fares from the point of trip-departure/sale, in local currency for that.

So, if I book GVA-ATH return, and I am looking at my flight options, I expect to see it all in CHF, and I expect to see 'marketable prices'.
i.e. 79.99 out, and 84.99 back, all in CHF.

Not prices that fluctuate all over the place for the return.

Now, easyjet is a point to point airline. However the same holds true. All easyjet fares have fare levels in the local currencies that easyjet supports on the route.

As a result, I would full expect to see 'rounded fares' i.e. 76.99 in currency A and say 23.99 in currency B, not 23.71 or 24.03.

This is NOT new.

The interesting bit is that easyjet is a purely point to point airline, so you can play with the quirks.

----

In this case, you are putting yourself onto the 'Euro' presentation friendly prices if you start with something priced in EUR, or the CHF friendly prices if you start with something priced in CHF.

No different to combining open jaws, etc, with legacy carriers.

-----

Other easyjet factors: The primary internal easyjet currency is GBP. However I believe they certainly used to have a second easyjet internal accounting currency of CHF - because easyJet Switzerland does run as a separate subsidiary from easyJet (main). It is the only one that is a separate subsidiary.
This has occasionally caused complicationtions with the workings of flights that touch switzerland.
But I think that was all solved a couple of years ago.

I think the only issue you are seeing is 'friendly pricing'.

----

First price you quote sets your quote currency, everything that follows is given in this quote currency, and easyjet have 'marketing friendly' prices in all relevant currencies at each price point.