The point is, you cannot state a "Fact" if it is not a fact..... The fact is we know it's 10km deep... so far... heck the only fact we know is that it sure aint over 50km deep as that is the thickest measured crust layer.
Well, this is because of taxes. On a large scale they have a reduction of 1% on total tax per product, while people rarely bother to pick up the spare change. Overall it leads to certain increase in profits.