Differential bound of the S-Box in GOST R34.11-2012
Differential bound of the sbox is an upper bound on the probability of the differential characteristic below, i.e.,
$$P(S(x_1) \oplus S(x_2) = b | x_1 \oplus x_2 = a)\leq \frac{8}{256},$$
for all $a,b.$

A differential used for cryptanalysis is assembled by joining different differential characteristics from different rounds. This results in a number of Sboxes being 'active', i.e., taking part in the overall differential.

However the probability of a differential would be upper bounded only by
$$(8/256)^k$$
where $k$ is the number of active sboxes that are part of the differential. If there are nontrivial attacks, this $k$ will be much smaller than 64.