I always took that to being a result of cracking the skull open hastily. I mean, Henry Frankenstein was not as obsessed with creating the perfect Adonis as Victor Frankenstein was in the novel.

But, by your title of this thread,  I thought you were referring to the shading that began on Karloff's cheek in order to make the monster appeal more cadaverous and then, slowly, with every movie, being lessened into the size of  a mole.