In particular the paragraphs where you talk about how larger companies want you to be a specific fit for a role. I've noticed that a lot of times my enthusiasm for working at a company comes out like "put me anywhere you feel is best, I won't let you down!" and it never occurred to me that some managers don't like filling in a lot of the gaps themselves.
So if that comes across as being indecisive then it must be a clear sign that I do not know how I would fit in. Sometimes I do get a manager saying they want to hire for a specific department and I just roll with it. And when you describe your experience, then it sounds like going with a "just put me in wherever" attitude ironically makes you sound disinterested even if that was not the intention.
As far as studying up on the data structures/algorithm specifics, I've been around enough big company discussions to have realized it. I go against the common grain in that I actually prefer whiteboard interviews over take-home test projects, too. Because everything about the interview is transparent and both sides essentially invest the same amount of time and progress with it.