Ah the years old question of false advertising. Love this one. So here's the deal. We are actually dealing with two terms here. We have MegaBits per second and MegaBytes per second. To be clear, these two units of measurement are not the same but do coincide in the networking world. Breaking it down to simple numbers we have Bits Per Second and Bytes Per Second.
approx 8.1 Bits = 1 Byte
So at the moment, you are paying for is 15 Megabits Per Second. If you divide 15 by 8.1, you will have about 1.9 Megabytes, just shy of 2 which is what you are actually paying for.
It is a stupid system actually, as these ISP's don't list what their unit of measurement is and technically it could be considered false advertising. It's also pretty bad when customer service for your ISP doesn't even know the difference. Even worse when you report them to the FCC and they said your ISP will send correspondence by mail in 10 days and it has been three weeks.
ISPs man, the soul suckers of the U.S