While this is sadly becoming more and more common, it is certainly not "true for all airlines". As far as I'm concerned, using different invetory for new tickets vs rebookings is an outright scam. I believe some lawsuits are ongoing regarding this very new practice and we're yet to see what the outcome will be. As things stand, for most airlines the fare difference really means fare difference and you only have to pay the difference between what you have paid vs what the price is right now. Either AFKL joined the scam too, or there's some other explanation, like repricing the return journey in a way the OP hasn't taken into the account.