Strange data plan speed on Nexus S

I have a Nexus S and a GPRS plan activated, from what customer services of my mobile carrier have told me is that GPRS will give you max of 10-15 kB/s download speed. In my Nexus S if i don't select "Use Only 2G" in "mobile network" setting i get speeds up to 80kB/S and the symbol in the status bar shows 3G symbol. As soon as i select the "Use only 2G Network" my download speed drops to 10 kB/s. I have used spedtest application to check speed. I have verified with my Mobile carrier and i do not have any 3G plan activated. How is this Possible, i don't want to get any kind of shock when i receive my Mobile bill :) Edit : My mistake, corrected capitalizations.