The 5-6-7 dancing mystery...

While darting across the uOttawa campus today I spotted a dance troupe doing their routine. As with swing dancing they counted off 5,6,7 and started their dance moves. The question I pose is why don't we say start off with 1,2,3 or 4,5,6. I also wonder what the origin of the of starting off with 5 as it seems that many other dances in addition to swing dancing use this protocol. Hmm...