Drives with perpendicular technology write bits more densely (more bit per centimetre or inch). That's why they have to rotate slower, to limit the number of bits per second, and to get more precision. So at the lower rotational speed, they actually read the same amount of data as say a 7200 RPM 100 GB drive.
The advantage grows when the drive gets full: All drives start filling at the outside of the drive where you have more space. When the drive gets full, you have to use space closer to the centre of the drive. Because you can't just increase the density, and the rotational speed is constant, you get fewer bits per second. A 100 GB drive filled with 80 GB of data is close to full and has to use areas that are close to the centre and therefore slow; a 250 GB perpendicular drive is still almost empty and uses tracks that are much further outside and still fast.
That's important for benchmarks. On an empty drive, 100 GB 7200 RPM is slightly faster. Fill both drives with 80 GB, and the 250 GB 4200 RPM is the winner.