As best I can tell, I Write Like is supposed to analyze sentence structure and other textual clues, rather than anything about plot, theme, or story structure, when it spits a name out at you. That makes sense; I don't even know how you could program something to do the latter.
Of the people I follow who've tried the program, the general consensus seems to be that it's an amusing novelty, but that it doesn't tell you anything useful. That is to say, that the connections between text entered and authorial doppelganger are opaque at best, and possibly not present in any meaningful (to a person) way at all.
As it happens, I have a convenient test of the program handy: Letters from a Senior to a Junior Changeling. That's a story I wrote that's explicitly based on C.S. Lewis's The Screwtape Letters, and which I went to some pains to match the writing style of. If there's anything to this program, matching Letters to Lewis (I don't know how many authors I Write Like can match a story to, but I know Lewis, at least, is on the list) should be a snap.
But let's be fair: The Screwtape Letters is hardly Lewis's best-known work. If the analyzer just has the Narnia books to compare my sample against, then it makes more sense that it wouldn't get that. So, what tipped it toward Lovecraft?
My first guess is vocabulary; Lovecraft was known for his
...Well, we're getting closer! In fact, Tolkien's not a bad guess at all; he and Lewis, while hardly possessing identical writing styles, are both of the same literary era and region (the two were good friends, after all). So, what's the difference between Tolkien and Lewis that it's seeing in my writing?
Well, assuming for the moment that Tolkien = The Hobbit + LotR and Lewis = Narnia, sentence length might be one. Letters is full of long sentences, while Lewis's books were written for children, and tended toward fairly simple construction (so did The Hobbit, but LotR not so much). So, I ran a find/replace of all the semicolons in my story to turn them into periods. Is that enough?
...Hmm, not quite. What if I replaced all the colons as well, and went through and deleted all but the first two phrases of every sentence with three or more?
...Nope, still not doing it. What if I split every paragraph with more than three sentences in half?
Okay, best I can figure is that at this point I've mangled the story so much that I Write Like has thrown up its metaphorical hands and said "I dunno, I guess it's epistolary? Dracula is epistolary."
Thus ends my foray into I Write Like. I don't think I learned anything useful, but it made for a fun fifteen minutes. Well, there is a bit of an epilogue to this adventure: after futzing with Letters for so long, I decided to run a few of my other stories through, to see if any of them were Lewis-like. They weren't... but four of the six I checked came back as Tolkien-ish. I'm still not sure how this thing does its analysis and if there's anything useful to be gained from it, but it seems to think I'm a pretty consistent writer, stylistically.