Level 10 being the max for skills in these sorts of series isn't that uncommon. Think of it like the rating system for series we have on here I guess? If you cap things at 10, it gives greater meaning to each individual level between 1 and 10. Whether you set that cap at 10, 100, or whatever else, it's all the same, really. A good example I can think of is the esper ratings in A Certain Magical Index and all its spin offs. Those only really go up to 5 originally, so the differences between each level can be kind of enormous.