title: why does conda update downgrade some libraries
body: <p>For m, on OS X, <code>conda update --all</code> often downgrades libraries - along with updating many.</p>
  
  <p>Is this usual?  Or something possibly in my setup?</p>
  
  <p>Earlier this year, it was pillow for many months. </p>
  
  <p>Surprisingly, today it was several of the HDF5 related libraries, numba and llvmlite. </p>
  
  <p>So <code>conda update numba</code> brings numba back to the most recent version, and so on with the other 8 libraries, but why doesn't <code>conda update --all</code> do this anyway?</p>