1) the leather is relatively unfinished and oils or heavy conditioners have been spilled / accumulated under the grain surface,
2) the leather is finished and the finish has been removed by abrasion or by chemicals that strip wax and finish. Any product that strips wax, has the potential to strip finish as well....any product. It depends on the nature of the finish. On some high end leathers such as crust, the entire finish is wax...think about it.
3) the grain surface has been broken through abrasion (scuffing, over zealous rubbing,)
The leather under the finish absorbs oil and conditioners and dirt far more readily than the leather that still has its finish. That's one of the reasons a finish is applied in the first place--to protect the leather.
And the leather below the grain surface absorbs oils and conditioners and dirt even more quickly.
It is unlikely that the dark spot will ever go away or return to the original colour.
Edited by DWFII - 3/1/14 at 7:29am