Why does it take a different time to generate a different pattern of the same length in vanitygen?

I've been looking at generating some vanity addresses today, and I found it surprising that the program generating the address predicts that it will take a different amount of tries to compute a vanity address of the same length, but with a different pattern. For example, generating "11..." takes 256 tries, while "12..." takes 23. Shouldn't it takes the same amount of tries to generate both of them, or am I missing some important detail?