Suppose a class B network: 128.16.0.0 Now if we make $2^6$ subnets first one will be:
$1000000.00010000. 000000|00.00000000\rightarrow$ mind the separator after 6 zeroes in third octet.
The first valid configurable address will be
$10000000.00010000.000000|00.00000001$
But people here are removing $2$ subnets (all $6$ zeroes and all $6$ ones) to remove base address and broadcast address. Doing so will render the above completely valid address unavailable. Consequently, we'll not have any network from
$10000000.00010000.000000|00.00000001$ to $10000000.00010000.000000|11.11111110$, and
$10000000.00010000.111111|00.00000001$ to $10000000.00010000.111111.11.11111110$.
And we certainly do not want to do that. So the subnet must remain $64$, and each of them will have $2^{10} - 2=1022$ hosts.