Thanks, and frankly I just don't know! and anyway it does not matter. I guess it is a kind of a parameter to show intermediate term health of the market. Also, 65 days directly relates to 13 weeks - a Fib number - but that's about it ! This is one of the methods used by Pradeep Bonde.
But anyway, no harm in using this I guess, as long as it directly points to the health of the markets, and we can always use the other internals for confirmation.