For each life tick of the bee queen, you have a chance to produce 0-2 product combs and 0-1 specialty combs. So the 5.1 mins (which is pretty clear is equal to 5 min + 0.1 min*60 sec/min) is there to give you some kind of average time information based on that chance. If you can reach the 2 products+1 specialty / life tick (with frame housings), then you have maximized the productivity of the queen.