More slow sectors with Hitachi 2TB than WD 2TB -- why?

Why does the MHDD surface scan diagnostic find more slow sectors with Hitachi drives than with WD Caviars? This is a typical result for WD20EADS and WD20EARS drives: < 3ms: 15,000,000 sectors <10ms: 640,000 <150ms: 0 <500ms: 0 >500ms: 0 but MHDD usually says something like this for 2TB Hitachi 5K3000 drives: < 3ms: 15,000,000 sectors <10ms: 9,700 <50ms: 25,000 <150ms: 300 <500ms: 1 >500ms: 0 All these drives pass factory diagnostics, and results are fairly consistent by model. Erasing the drives completely made no difference. There have been no problems with them, and I’m just curious. Among the 4 brands of HDDs I’ve tested, only the WDs show no sector needing over 50ms to be read.

When you measure the transfer rate, is it the same?

The transfer rates are about the same between the drives that show no sectors slower than 50ms and those that hundreds of such slow sectors.