Looks like the method involves selecting a random number within a specified range defined by the number of bits of entropy. E.g. each output (or Bitcoin address) corresponds to a private key that was generated by selecting a random number within the range of (2^n-1) to (2^n)-1 , where n is the number of bits of entropy designated for that particular key. So that range ensures that the selected number has exactly n bits when represented in binary form matching the pattern of increasing entropy with each subsequent address.

Each subsequent address uses a private key with one additional bit of entropy than the previous one. The selection of the specific private key within that range is random. To generate or guess the next unclaimed output's private key (e.g., for the address with a 51-bit key), one would theoretically need to try all possible values within the range from (2^50) to (2^51)-1. By using a sufficiently high number of entropy bits, a quantum machine might be needed to brute-force through all possible keys in this range.