OK, my guess is its an attempt to not bother with stating a tolerance on the RPM figures. There are 4 figures in the datasheets, 7200, 5640 (which is a bit odd, but fine) and 5400rpm.
There are some notes however, the 12TB Plus' actual speed is 7200rpm but may report 5400rpm as its transfer rate is slower than the other 7200rpm drives in the range; so I suspect they are trying to compensate for that needless bit of complexity they've created.