The problem is that modern hard drives use a zone bit recording technique that allows different read speeds depending on where the data is located. Data on the outside of the drive is much faster than data recorded on the inside. HD Tach reads from areas all over the hard drive and reports an average speed. It also logs the read speeds to a text file that you can load into a spreadsheet and graph to visually read the results of the test. In addition to sequential read, HD Tach tests the drive's random access time.
Random access is the true measure of seek speed.
Many drives advertise sub 10 millisecond seek speeds, but seek speeds are misleading. Access time is the time it really takes to read data, not just the time it takes to move the head to the proper cylinder. To calculate access time the software must read a single sector off of the hard drive. A 7200 rpm drive has a rotational latency of 4.15 ms on average. A 4500 rpm drive has a rotational latency of 6.67 ms on average.