Why has the Earth's average sea level risen 'only' by nine inches after one degree C increase in temperature?
You should take into account isostasy. As there's more water in the oceans, the crust underneath bends under the weight, creating more room for it. At the same time, continental crust heaves above the oceanic, hence the small value.
A question here to be careful about is where the location of the measuring point, as it very well may not be stable.