**This is a bug (or an imperfection) of `ColorData["VisibleSpectrum"]`**.

Others have delved into its root, but in my answer I simply will make it clear by a comparison with an experimental, known single wavelength color<sup>\[1\]</sup>: sodium’s D-line at 589.0 nm.

Here is Mathematica’s idea of the color of this wavelength, using default settings (no color profile conversion applied, default MMA settings on my MacBook Pro, Mac OS X 10.8.2):

![enter image description here][1]

Now, here are spectroscopic observations of this same color:

- [Fabry-Perot interferometer][2]

  ![enter image description here][3]

- [Michelson interferometer](http://www.unit.eu/ori-oai-search/friendly/thematic-search.html/id--2403/submenuKey--thematic_unit/menuKey--unt/first_index--40)

  ![enter image description here][4]

In conclusion: **sure, color conversions are tricky… but here, Mathematica is well outside the margin of error :)**

 

 

 

---
\[1\] Well, almost single color… the two lines are not that far apart. 


  [1]: https://i.sstatic.net/n60hY.png
  [2]: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fabry4.html
  [3]: https://i.sstatic.net/6ThLA.jpg
  [4]: https://i.sstatic.net/bZL4O.jpg