Actually, it's because the P3 has more power per clock than the P4. Sad really, but the reason they went to a P4 was because they had the chip maxed out. The P4 is actually a cut down version of the P3, as such that they took out some of the processing power as a sacrifice for speed. Then they use the "mhz myth" to make up for it. It is true that the P4 has other advancements and such, but it's actually true that a P3 is better than a P4 at the same clock. It's like comparing a Willy P4 (256 cache) to a Northwood P4(512 cache), the northwood will win every time.
This is all proven that a P3 head to head with an AthlonXP at the same clock, the P3 would win, whereas a P4 head to head with an AthlonXP at the same clock, the XP wins.