There are quite a few contradictions in the show.
Sheldon has a memory capable of remembering where every piece of food on his plate was, yet later in an episode he mistakenly drinks from anthers cup of water and is fooled into believing he did so again.
He likes Babylon 5, oh wait he hates Babylon 5.
First episode he and Leonard goes to give sperm, later on in another episode he mocks the notion of someone doing so.
Such things happen.