I'm not sure why they recommend such a value but I do know there are a lot of different values floating out there as the recommended alkalinity value for reef aquariums. That may be the value they've had success with in their R and D efforts whereas other companies have had success with other values. There's not really a whole lot of sound evidence as to which level is best and different levels of other water chemistry elements can interact with and influence alkalinity too. I'd just try to stay stable at a level of around 8-10 dkh and closely watch your corals to see how they are doing, they are the best indicators for water chemistry in my opinion.