Backlogs, over-/understock and supply chain issues.
If
GW's biggest supply chain time constraint is the books, being printed and shipped en-mass from abroad, that dictates how many of a thing they can put on shelves.
GW makes the models themselves, and can create more models for a box pretty easily, compared to printing more books.
So, if you treat the Leviathan box as having two halves, the models and the books.
GW orders X books from the international printers, and they know they will arrive around a particular date.
They then book themselves some time in the model factory to make the amount of models for the first batch, to coincide with the books arriving.
Throw in an extra model-making window at pre-order time, to make more if the pre-orders go too wild.
If the pre-orders scoop up far more boxes than they made, they can make more, up to the X books they had printed and shipped. They cannot make more boxes, as they need to order more, and wait for them to arrive.
But if the per-orders hit targets, and do not go over by too many, they make that many more models, plus a few for the shelves, and have a pile of books left over.
Once the Lev boxes are all out to players, stores (and scalpers), they can sell the rest of the books off separately,
But, if they put a Leviathan cover on it at the printers, they're stuck with a pile of Leviathan books they won't sell as single-item rulebooks.
Do covers come pre-stuck (as I'd expect them to) with Standard rulebooks coming later?
Do the covers get stuck on afterwards? (I really can't see that happening, but would explain a lot of the stock issues).
Very off-topic for the
OP's question, but am trying to understand
GW's exploits.