If you know that a key will be less than and greater than a maximum and minimum, it limits the values that the higher bits can have (else it wouldn't fit in that range, now would it  )

For example, say that you know that a private key is between 115 and 120 bits long. That automatically means, since private keys are 256 numbers, that the bits after the 120th one (assume 1-based counting for simplicity) are all zero.

Because the ranges in these puzzle addresses are only ever going to be between (range-1) and range bits (i.e. that particular bit will always be set), it is straightforward to determine the higher bits that are zero or in a certain range like 0-4, 0-C, etc. that's what makes brute-forcing possible.