Because 2^57 has 58 digits (in binary form '1' + 57 '0'). Then 2^57 could be the key #58, not #57

key 1 -> 1 bit :  1  ( from 2^0 to 2^1 - 1)

key 2 -> 2 bit :  from 2 to 3 (from 2^1 to 2^2 - 1) : 10 or 11

key 3 -> 3 bit : from 4 to 7 (from 2^2 to 2^3 - 1) : 100 or 101 or 110 or 111

key 57 -> 57 bit : from 2^56 to 2^57 - 1

key 58 -> 58 bit : from 2^57 to 2^58 - 1

If you look at

key #3
.....0000000000000000000000000000000000000000000000000000000000000111 (to address 19ZewH8Kk1PDbSNdJ97FP4EiCjTRaZMZQA)

and at the #4
.....0000000000000000000000000000000000000000000000000000000000001000 (to address 1EhqbyUMvvs7BfL8goY6qcPbD6YKfPqb7e)

key #3 = 2^3 - 1 = 7
key #4 = 2^3      = 8