It comes from the way the hides were originally treated in the different countries. In Spain oils of rose, verbena, sandalwood, clove and vanilla were rubbed into the leather to cure it. This gave the leather a particular smell which lasted with it, and that smell became known as the Spanish Leather scent, or Peau d' Espagne.
In England we tended to cure leather with either oil of cloves or brine (heavily salted water used to preserve meat and fish), over the years floral waters were added to the English brine to scent it, and English Leather can now really mean any scent that is woody and or green, but the Spanish Leather is a particular scent which varies a little from maker to maker.