Gf wants to split expenses 60/40 and have me repay her for expenses we split 50/50. Fair?

I (27M) moved in with my girlfriend (26F) last August when we had jobs that paid roughly the same, and we agreed to split all our costs 50/50. A couple months later, I got a new job with a considerably higher salary. We kept splitting our costs 50/50 for several months -- it genuinely hadn't occurred to either of us to change anything -- but last week she asked me to take on a higher proportion of the cost, 60/40, because it's unfair for her to pay the same as me when she makes much less. She has student debt and no savings; I have strong savings and no debt.
I didn't disagree. This seemed fair, and I was happy to oblige. But then she asked me to repay her for the past few months since I got the better-paying job to make up that 60/40 balance.
Does this seem fair? I recognize that, if we agree the arrangement isn't fair now, then it wasn't fair before either. At the same time, we'd agreed to split everything 50/50 before, and it seems strange to apply our new agreement retroactively.
I don't think anyone here intends harm. I'm not trying to be cheap; she's not trying to unduly take my money. I think we're both trying to do what's fair. But I'm curious what you think.
TL;DR: Gf and I split expenses 50/50. I started making more money, so she asked to split expenses 60/40. I'm good with that, but she's also asking to retroactively apply this 60/40 split so that I repay her for previous 50/50 expenses. Fair?


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