The difference is in the knowledge that the guess is right. Guessing 14 bits has a probability of 2^-14, but unless there's a weakness you have to brute-force the other 50 bits *for each guess* to find out if your guess is right. So recovering 14 bits is 2^64 units of work. (I'm assuming 64 bit key length.)
If on the other hand you can recover 14 bits in 2^14 units of work, you can crack the key in 2^50+2^14 steps, considerably lower than 2^64.