Bluetooth advertises around ten metres of range, measured in an open space with nothing in the way. Real rooms contain people, walls and furniture, and the 2.4 GHz radio band it uses is absorbed very effectively by water — which is most of a human body.
That band is also shared. Wi-Fi, microwave ovens, wireless keyboards and every other Bluetooth device nearby compete for the same airspace, and in a busy café that competition is fierce.
Devices cope by re-sending lost data, which you hear as a stutter. Reduce the obstruction or the competition and the stutter disappears.