We're talking about legacy bitcoin addresses here. Addresses are a base58 encoding of the underlying public key, with a checksum on the end. The public key is a binary number, which is always 65 bytes long. However, when encoding it, leading zeroes are discarded. Hence, if you randomly pick a private key that produces a public key with lots of leading zeroes, your base58 encoding will be short. See bitcoin.stackexchange.com/questions/36944/what-are-the-minimum-and-maximum-lengths-of-a-mainnet-bitcoin-address
