Hard Versus Soft
In the traditional publishing world, a book has approximately 6 weeks to make an impact before the brick-and-mortar booksellers return your books to the publisher to make room on the shelf. It used to be longer (and there used to be a chain store called Borders, too), but changes to the tax code 20 or so years ago treated books the same way it treated screwdrivers and wrenches and changed the taxation on warehoused retail products. This incentivized bookstores and publishers to churn books rather than keep them in a warehouse or on shelves.
The result of this is that from the day your book is launched (traditionally a Tuesday), the writer needs to hustle and sell in order to move as many books as possible for a 4-6 week period from launch. That's why review copies needed to be to newspapers and review outlets 4 or 5 months ahead of launch and why your book signings etc were all so dramatically tied into the launch date. You were trying not just to make a big splash, but to convince booksellers that you were worth stocking, and as a result, your publisher (who was already probably making decisions on your next book based on pre-orders anyway).
This is more or less true of ebooks by traditional publishers although it's probably much fuzzier, because of the simple nature of ebooks, i.e., they don't go out of print and there's no particular storage issue at warehouses.
Which is where a "soft launch" with ebooks comes in. I'd be delighted if I published a book and it sold thousands of copies on the first day (or ever, for that matter), but I do understand (intellectually, if not always emotionally) that ebooks just keep selling. I might sell a few dozen of the new book the first couple weeks, but unlike with traditional publishing where those numbers might peter out, ebooks SEEM to continue selling steadily month after month with occasional peaks and valleys tied into whatever promotion I'm doing or the phases of the moon or the launch of another book or another change in the phantom logarithms created by Amazon's computer wizards.
Whether things will change remains to be seen. I recently applied to BookBub for a promotion slot and was turned down, so I will continue doing my drip-drip-drip Chinese Water Torture thing—I tend to link my free and sales promotions to the launch of a new book, but we'll see, I'm a number of months from a new book.
I guess the point is to be aware that in the marketplace, ebook sales BEHAVE differently than traditional paper book sales. There are a lot of reasons for it. I'm sure one is simply that if I see a new book come out on ebook that is by someone I don't know but looks interesting, I will download a free sample to remind me at a later date to check it out, rather than the oh-that-looks-cool-gotta-buy-it-now thing.