It should be started from zero as I understand. Or from 1.
But according to this suggestion it is not started from zero, it was started from something, or from some max value may be:
if (counted_bars > 0) counted_bars --;

As Codersguru described before "pos" is the number of time for the loop to work.
Because:
int pos = Bars - counted_bars;

It means if counted_bars is equal to the total number of Bars in the charts the loop will be finished. So, counted_bars should be started from some miminum and finished with some maximum.

May be I do not understand something.