Rule 7.05(f) [Each runner can advance] "Two bases, if a fair ball bounces or is deflected into the stands outside the first or third base foul lines; or if it goes through or under a field fence, or through or under a scoreboard, or through or under shrubbery or vines on the fence; or if it sticks in such fence, scoreboard, shrubbery or vines;" According to Baseball Reference, only when there is fan interference can the ump use his judgment on whether or not to award the runner that extra base.