I think Athalons use faster chips than Pentiums. An Athalon 3000 and a Pentium 3GHz have the same resulting speed, but the Athalon only runs at about 70% of the clock rate. The Athalon uses slower chips that are more efficient.
My Athalon 1800 has a clock speed of 1.15GHz, but it runs like a 1.8GHz machine.
