Stellar Distance Question?

Prior to HIPARCOS the main methods of determining stellar distances were measured parallax and interpolation through the HR diagram. Parallax is only accurate +/- 10 per cent or more, and cannot be figured past about 600 lightyears. Interpolating from a star's position on the HR diagram is even less accurate, although usable out to as far as we can see individual stars. HIPARCOS, however, was accurate out past 1000 ly, and better than 10%. Thus you may be looking at pre-HIPARCOS results or estimates using another source. Giving 1500 ly for all three Belt stars is grossly wrong though.