Absolute/apparent magnitude and distance for HIP31978 inconsistent? EDIT: I am still following up, but here's the latest response from Wolfram:


  Paclet servers are updated and optimized for each version of
  Mathematica, so I suspect you may be working on an older version which
  we no longer update. Many times we get reports that data from the
  paclet data for version 9 was incorrect, only to find that it has
  already been fixed for version 10.


According to Mathematica and several other sources, the
known information about HIP31978 (also known as "S
Monocerotis" and "15 Monocerotis") includes:


The star system is about 101.06 light years from our own.
The star's absolute magnitude is -2.79 (some sources say even lower).
The star's visual magnitude from our solar system is 4.66.


The star is slightly variable, but not enough to explain
the following discrepancy:

If the star's magnitude at 32.6 light years is -2.79
(definition of absolute magnitude), it would be about 9.6
times fainter at 101.06 light years (it's actual distance
from us). However, that's only about 2.5 magnitudes
fainter, nowhere near enough to bring it down to magnitude
4.66.

What's happening here, and is this a one-off special case,
or does this happen a lot?

For reference, my calculation of the apparent magnitude:

-2.79 - Log[100, (32.6/101.06)^2]*5 == -0.333192
The absolute magnitude you quote appears to correspond to the Hipparcos parallax distance estimate, which is $\sim 1000 \mbox{ ly}$, not the $\sim 100 \mbox{ ly}$ distance you have used.