It is their standard amount for all beers. Unfortunately as we all know, different styles have differing carbonation levels so while it might suit lagers and pale beers, it doesn't suit stouts and the like. And then there is also personal preference that comes into it - you're brewing for you after all.
If I was you, I'd bottle a few of the bottles with different levels of carbonation and see which one you like the best, then you can use this going forward for that beer style. For the majority of them though, use the lower amount because a stout is more enjoyable with less carbonation than 8g/L will give, although I personally prefer it more around the 5g/L myself.