PKCS#1 v2.1/RFC3437 defines the OAEP padding mode for encryption and the PSS for signature padding. Both schemes are based on the generic constructions not specifically tailored to RSA by Rogaway & Bellare.
One of the alterations done for RSA-OAEP is, that the actual padding is only done to a length of $n-1$ (where is the octet length of the modulus) and a single zero byte is prepended to the whole string. This is done, since a message padded to $n$ bytes could be larger than the modulus, thus creating an ambiguity when decrypting.
My question is: The same argument could be raised for RSA-PSS, but here no zero byte is prepended to the padded hash. Why is that not necessary for signatures?