Not that it isn't true but there are some misconceptions along the way for most. Easiest way to explain is, from exchange to you the speed kinda drops off as it travels the distance. So for 1-2km you can easily get 4-6mbps if you have good stable connection. For anything above that its really where
ADSL2+ is the blame more than anything. So to compensate with the loss in speed with distance they need to provide higher data rate than the actual speed which makes it difficult at certain point when the distance is far too much as too much data rate/higher bandwidth would just make your connection very unstable. They can only give you the data rate to a point where you don't keep disconnecting.
But the distance between exchange to you isn't really the road distance per se. Also take in account how many places it has been joined to reach you which causes connection instability.
Distance from Exchange to you (the MTNL box in your building) could be less than the road distance as the underground wiring by MTNL is quite an old good underground infra and city around us has developed differently.
So if your present 2mbps connection is stable, just upgrade to higher speed this month and see how it is. As I said before higher data rate would be provided to you to bring you close to speed you pay for.
For my 10mbps plan, my data rate is between 11-11.5mbps. This gives me speed between 9.5-10mbps.