I haven't checked, but I assume a single Bitcoin client doesn't try to hash the same value more than once. So if I hash a value and generate an unsuccessful hash, the client won't try to hash that value again. However, independent clients have no such knowledge of what values other clients have already hashed. Therefore 1 computer running at 1000 hashes a second will find a successful hash more often than 10 computers running at 100 hashes a second because those 10 computers might be trying to hash the same value that will generate an unsuccessful hash.