In 2001, AMD launched the Athlon XP Palomino. To counter Pentium 4, a CPU with high clock speeds but lower IPC, the XP was given model numbers based on performance that lined up with the P4's frequency. At the time, clockspeed was everything, and there was a lot of controversy around the differences in IPC. I even remember a few bad analogies being made from people trying to explain that there was more to CPU performance than clockspeeds. Something about a kid having to move his legs faster to keep up with an adult. Naming conventions pretty much stuck around with AMD ever since the Athlon XP.
Intel didn't introduce consumer CPU model numbers until around 2006 with the Pentium 4. Netburst had pretty much failed by that time, so in order to prepare us for the lower-clocked but far superior Core architecture, Pentium 4 needed to adopt lesser model numbers than Core, which was clocked lower but performed way better. I guess in a weird sense, our model numbers today are still based on Netburst.