Does "single barrel" mean every bottle tastes the same?

"Single barrel" means every bottle in that release came from one specific cask, rather than being mingled with dozens or hundreds of other barrels to smooth out differences before bottling. That's the entire claim the term makes; it says nothing about consistency between different single-barrel releases.

Barrel-to-barrel variation is real, and for single barrel bottlings it's the point rather than a flaw: two barrels filled the same day from the same distillate can still age differently depending on where each one sat in the rickhouse; floor level, wall-versus-center placement, even which way the barrel faced. That's why single barrel labels often print the specific barrel number, warehouse, and rick location: a distillery's way of saying "this exact barrel," not "our standard flavor."