Just speculation; the book may not be re-printed (or, with no definite plan to), so it is discontinued to HLJ. Because it is not something they can restock, so it's not stated as out of stock either.
HMV just hasn't sold out the stocks yet so it is still in stock.
BTW, issue #23 is a HOT issue; I used to purchase my Macross Chronicle in a shop and their MC stocks usually last for a week or more. For this issue it was sold out in 2 days.