It is simply because he has gathered more facts from each of these three sources,--experience, hearsay, print.
To those who have science enough to appreciate the above distinction, I am very willing to admit that in all my tales I use a vast deal of heterogeneous material, which in a life of study I have gathered from men, journals, blue-books, histories, biographies, law reports, etc.

And if I could, I would gladly specify all the various printed sources to which I am indebted.

But my memory is not equal to such a feat.

I can only say that I rarely write a novel without milking about two hundred heterogeneous cows into my pail, and that "A Simpleton" is no exception to my general method; that method is the true method, and the best, and if on that method I do not write prime novels, it is the fault of the man, and not of the method.
I give the following particulars as an illustration of my method: In "A Simpleton," the whole business of the girl spitting blood, the surgeon ascribing it to the liver, the consultation, the final solution of the mystery, is a matter of personal experience accurately recorded.
But the rest of the medical truths, both fact and argument, are all from medical books far too numerous to specify.

This includes the strange fluctuations of memory in a man recovering his reason by degrees.