It only matters if the act is pure altruism—that is, the giver gets no benefit, or even sacrifices, for the recipient.

For there to be a real distinction you don't need there to be no benefit to the giver, just less benefit than the giver could have gotten in other ways. Yes, a fully anonymous gift to someone you'll never meet does make you feel good, but because of scope insensitivity the difference between donating 10% and 20% of income has incredibly little benefit to the giver.

If I see someone donate 20% when 10% would have almost the same benefits to them, and that money could instead if spent selfishly buy other things they would have enjoyed a lot, then their claim that they're doing it because it's the right thing to do seems pretty plausible.

(One counter is that if they will be unhappy donating less than they believe they ought to, then there is a real difference between 10% and 20%.)