That's strange. I would assume 7200rpm drive would be costly to produce compared to 5400rpm. What possible reason could they have to falsely advertising which could be a loss.
Producing 7200 RPM drives likely is more costly than 5400 in isolation, but running two full production lines itself is costly, when you could just run one and "fix it" via software.
Plus it allows you to vary the number of 5400/7200 RPM drives you "produce" to exactly what the market wants. Just keep them unlabeled in a warehouse as orders arrive: label, firmware, and ship.