How to interpret error of absolute magnitude of a star given data from wikipedia?
Spica (&alpha; Vir) is a binary star.
The Wikipedia article cites Herbison-Evans et al. 1971, whose Table III says:


  Absolute magnitude of primary (Mv1) &nbsp; -3.5&plusmn;0.1
  Absolute magnitude of secondary (Mv2) &nbsp; -1.5&plusmn;0.2


So the confusing "-3.55 (-3.5 / -1.5)" can be interpreted as "combined (primary / secondary)."