I think our belt might be made of adamant.
Here's some stuff from Dwarfs 8e:
So basically, the reason why you don't have more than three runes or more than one master rune is because the sheer amount of magic power would put way too much strain on the inscribed object otherwise. This applies to armour runes too, and armour runes can only be inscribed on gromril armour.
However, I suspect that that's only for
mithril, the most common and basic sort of gromril. According to
Lexicanum, adamant is a more advanced form of gromril. It's so magically inert that someone wearing adamant armour is outright immune to magic attacks, yet adamant can still be enchanted. Between the fact that it's so utterly resistant to magic yet still capable of being enchanted, I think that the reason why we can have so many runes of such power on our belt is because the belt's made of adamant, which can take the strain.