It's not, it's really not. He's wearing an appliance.I think that's just the way the shadows are playing on his face.
Look at Eric Bana's "un-made-up" face. Look at this picture. Look specifically at the region between his nose and his brow. In the picture, there's a big, broad "outswell" from the nose to the brow which the REAL Eric Bana (without makeup) does not have.
This detail is something which all Klingon makeup since TMP has had, however, to varying extents. This is why the "is this guy a Klingon" conversation was taking place... and the "what, exactly, IS this guy" is STILL taking place (albeit it's pretty much confirmed he's a Romulan... the real issue is now "what does that mean in the context of this character... 'cuz he sure doesn't look like he's the same race as Spock!)