Wikipedia (emphasis mine):
"The Great Deflation or the Great Sag refers to the period from 1870 until 1890 in which world prices of goods, materials and labor decreased.This had a negative effect on established industrial economies such as Great Britain while simultaneously allowing incredible growth in the United States which was just beginning to industrialize. Deflation has historically been more associated with recession, than growth, but this is one of the few sustained periods of deflationary growth in the history of the United States."
http://en.wikipedia.org/wiki/The_Great_Deflation
This clearly refutes your statement:
"Deflation is almost invariably a sign of economic distress."
Perhaps deflation isn't traditionally associated with growth, but it clearly can be and has been in history. Note 'one of the few'. Deflationary growth isn't that rare of a phenomenon in history, though its rarity will be inversely proportional to the prevalence of central banking, monetary expansion, and economic authoritarianism. The market's had an uphill battle.
In general, deflations associated with recessions will also be associated with reckless monetary expansion. Including this form of deflation in statistics can be misleading. Its cause is entirely different from the cause of gradual, moderate deflation as the natural result of an expanding economy with limited currency.