I found that on route like SEA-JFK it's related to fare difference, when fare difference is under $400 then it's 1 cert, $400-$600 3 certs, >$600 4 certs or even no move to mint allowed.
But on other routes with more flights to NYC per day, it's usually 1-2 certs. I guess the scarcity of the route matters more than price difference.