I fly LHR to Canada (YYZ, YUL, sometimes YVR) about twice a year. In previous bookings, I've booked Flex fares because it said at the time of booking there wouldn't be any change fees. However, more recently I went to book a flight and I found the flex option had change fees, and that it was the Comfort fare I needed to book for no change fees.

I was a bit surprised by this and went to look at the official statement on the fares and it says that Flex fares are subject to a change fee. Furthermore, it seems to have always been the case looking back on some older Reddit posts.

However, looking at some of my old booking emails it seems that Flex doesn't always have a change fee. Here's a trip I took over the most recent Christmas period:



And another one I took recently LHR-YVR:


Is there a rhyme or reason as to why some Flex fares have change fees or others don't?