Of course they have fare rules.
Honestly this sounds like an agent in a situation s/he has no training for. Finnair has filled up the ranks with staff without prior knowledge and a customer service agent with limited (or no) travel agent training could mistake "fare rules" for a set of rules that describes how the price will change, like "change X leads to price increase of 10% and change fee 50€".

Perhaps even agents aren't given access to the actual fare rules, and it might been that they have never seen what is inside the GDS. I suspect some of them just have booking tools similar to what the end user has on the site. Thus the only solution the agent has been taught (or figured out) is to do the rebooking and discover the new price. Naturally, if you spend your day doing such re-bookings it would look like the new prices are basically random and hence the "there are no written rules for fare difference".

Fare rules used ot be accessible on site during the booking, so it is a concious change Finnair has made to remove them. It has been said they "are working on getting them back".