Actually, there is no difference between Flex and Saver fares with respect to flexibility. I have seen Flex fares with higher change fees than Savers.

I bought a ticket this week which has IDENTICAL fare conditions outbound and inbound, with the sole difference is that the outbound (B class) is Flex and the inbound (K class) is Saver. Change fees ($100), refundability ($150 penalty), restrictions (no min/max stay), validity (one year) are all the same.

Flex and Saver are simply brand names that have been given to fare classes and have no relation to the actual meanings of the terms "Flex" and "Saver".