HSDPA speeds for 2G plans

I've been subscribing to BSNL's 2G internet plan for the past 1 year. Right from day one,whenever I connect to the internet, 'H' is displayed on the top of the screen. I understand it stands for HSDPA or 3.5G and that a lot of people are getting 3-3.5G speeds on 2G plans. Can anyone explain to me how this is possible? If we are getting such amazing speeds at 2G cost, then what is the need for subscribing to 3G plans?

In any case, bsnl has unified plans for 2G and 3G.

yeah so your recharge gives you 3G data whenever you have the network else reverts back to 2G. the pack remains the same.