According to WD’s deceptive (?) marketing-speak, “for each drive model, WD may use a different, invariable RPM.”
Here is the whole paragraph:
IntelliPower – a fine-tuned balance of spin speed, transfer rate, and caching algorithms designed to deliver both significant power savings and solid performance. For each drive model, WD may use a different, invariable RPM.
This means that each model runs at a fixed speed. It is not variable.
AIUI, a drive’s heads fly on an air bearing generated by the spinning platters. If the rotation speed were to vary, then so would the flying height, leading to variations in read/write amplitude. Therefore the heads are aerodynamically tuned to a particular speed, and that speed is tightly controlled.
The following thread discusses two methods to determine the actual RPM of the drive:
The simplest method is to measure the width of the access time graph in HD Tune’s read benchmark. The OP’s drive spins at 5400 RPM. Therefore the spread of data points will be about 11 msec, ie the time required for one revolution of the platters.