This reminds me of how, to make a whole lineup of Intel chips, they just make the highest-price model, and then disable features to create the lower-price ones. Or how shorter-range Teslas have the same battery capacity as longer-range ones--they just disable using the extra capacity in software.
In this case, the "5400 RPM" drive runs at 7200 RPM, but WD presumably makes it (either via software or hardware) perform worse in other ways to compensate, so that its performance is really only competitive with actual 5400 RPM drives.
That way, they can justify the lower price point without actually needing two separate manufacturing processes for each kind of drive (like how Intel saves on having to redesign each chip in a lineup).