MIPS Assembly Language $($Kann CH$7,$ for reference$)$

In section $7\mathrm{.}8\mathrm{.}3,$ it was said that a branch could access addresses that were $-8191..\mathrm{.}8192$

distance from the current $pc$.$ However, the $2\text{'}$s complement integer has values from

$-8192..\mathrm{.}8191.$

Why the discrepancy between the value of the $2\text{'}$s complement integer and the size of the

branch?