you mean Eastern ?
You can see the west ridge with Peña L´Ombre. It's the second option to reach the summit (we walked this ridge as descent). Can you see Perdido on right side? if you're on the summit and that's Perdido...this is the western ridge.
The eastern ridge is the shortest approach across a very small coll at S.E. of the summit.
I see what you mean, because the C.M. is oriented from NW to SE. But even if we watch more towards the West than the East, as seen from the map we are on the right edge of Castillo, no ? (otherwise no Monte Perdido on the picture)