Two major reasons: the number of stars isn't really seven (it's many more), and they all have different brightnesses, so that you see more or fewer of them depending on viewing conditions. The brightness of stars is measured on the magnitude scale, where the most prominent stars in the sky are around 1st magnitude, and the faintest you can see with your eye are about 6th. ... I think this makes it quite likely that the list will be truncated at Taygeta, giving six visible stars. ...

... 2.3 Data acquisition Spectra generated by the Multibeam correlator contain the usual structure seen in spectra from other H i instruments: a bandpass spectrum which is the sum of the sky , ground and receiver temperatures multiplied by the product of the filters in the receiver chain; superimposed on this is a noise ... Note also that sources extended much beyond ,40 arcmin can be imaged poorly in HIPASS images because of the bandpass sidelobe effect described in Section 3.3. ...