In justice, redress can only follow a breach of a rule. In social justice, a claim for a redistribution is not grounded on any rule. It is simply a good try. Whether it is satisfied depends on the politics of the time and place. Where political decisions ultimately depend on majority vote, claims in social justice that ostensibly or really favour the poor have a better chance of being satisfied than claims having no income or class bias, but this will not necessarily be the case. Measures favouring the poor at the expense of every body else will usually command sympathy, but sympathy does not make them less unjust.
Anthony de Jasay, Perspectives in Moral Science 2009 [no link available]