This may seem obvious, but it’s important.
Jane and Frank work weekdays at a restaurant. One day, their boss offers them both an extra shift – a one-off – at the weekend. Jane accepts; Frank declines.
Jane will earn not just more, but infinitely more than Frank on the weekend in question. Presumably we are happy with this inequality.
We hope that Jane thinks the money she earned (perhaps together with the gratitude of her boss or colleagues) was worth her time and effort.
We hope that Frank will treasure the memories (or results) of the time he spent with his children / playing football / managing his investments / volunteering for a charity / writing his novel / in bed with a hangover.
This inequality-generating event could be repeated the next weekend too, even every weekend for the rest of their lives, causing ever-increasing inequality between Jane and Frank but no change in the justice of the situation each time.
Conclusion:
High levels of inequality may sometimes be undesirable, and injustice is always undesirable (or rather, it’s always wrong by definition), but they are not the same thing.