I would guess that it depends mainly on Prima (maker of the guide), because I think that they are ones that responsible of producing and selling guide books.
I just pre-ordered the hardcover and while the listing displays a release date, the order page says it will notify me by email when the book is available to ship. Which is slightly concerning, I thought pre-order items generally are advertised to ship on release date. It's probably nothing, I would be shocked if this guide sells out before release. Well, shocked and elated, I want PoE to do well.
If you ordered it from Amazon, then that is quite normal as they don't want to be hold accountable if there is delays in production, which is why they don't promise shipping dates before they have product in their stock.
I hope your are correct, but my sixth shipping sense tells me something is up with supply and demand for this book.
The expected delivery for the other one, the art book from Dark Horse, is day of release, March 24, with a four. April 1 is when the order page for that book says it'll come. Almost a month before the strategy guide. Did I pay for two-day shipping ... no, I need beer money.