it's all in the wiring/cabling between the customer and provider. you can have all the best equipment at both ends but then the wiring in between is crappy with tons of signal loss, then you won't have an efficient/reliable connection.
and with variables like old neighborhoods with old cabling or areas with high subscriber density sharing the same node, can all affect your bandwidth. some areas are better than others, some aren't. it's all in the luck of the draw.