"Because humans have 10 fingers"
Wow, i- i'm genuinely mindblown by this simple clever revelation. If there is actually a very grounded and logical reason as for why human society in a fantasy setting in fiction uses Base10 Numbers, then this is it.
I was so used to fantasy manga that i guess my disbelief suspension is on a level where i can be amazed by real-life common sense like this, lol.
@Douteigami, i like to think that it's more like the Clerk Girl want to test Saitou if he can detect the sword's secret too, and then happy to know that Saitou did managed to, because this means Saitou to the Clerk Girl is like that friend where you can share your hobby with, which in this case seems to be "inspecting/appraising things". This also provides a more logical reason as to why the Armor Girl is getting jealous, because she knows that the Clerk Girl is having a good time discussing worth of items with Saitou.