Even Tallet agrees it wasn't there from the beginning. He clearly acknowledges that it may have been in that Gallery for untold years, but when it was finally moved from the gallery, which he presumes was rolled up in someone's pack, it was accidentally dropped outside the gallery.
A more detailed account appears in MIFAO 136. Tallet believes that what happened is that, originally, about 15 papyrus scrolls, at that stage still rolled up and tied by a string (ficelle), were placed at the bottom of the cavity formed by the two slabs sealing off the gallery. This took place when the gallery was last used. The rolls of papyrus could have been deposited only after the gallery was sealed. It is impossible to tell whether the rolls were just thrown in the cavity, or might have been carefully placed there. The remains of scraps of fabric suggest that, originally, the scrolls might have been in a linen bag (4) (probably the "pack" that you mentioned earlier.) However, Merer's logbook itself probably originally consisted of five scrolls (14).
So if Merer didn't put it there, and if it was originally held in storage in the gallery, then please tell how long it's been there and how you know that.
The papyri weren't stored. They were either deliberately thrown in the cavity, or deliberately placed in it. On a subsequent occasion, they were disturbed and dislodged - which actually helped to preserve some of them (5). The rolls left at the bottom of the pit deteriorated because of water damage. But other fragments, in a part of the pit that was much drier, survived much better.
It should probably also be added that papyri scraps (including those that are accountancy records, rather than Merer's logbook) were found in several different locations: e.g., G7, G17, G8 and G12.
... maybe you also can shed light on how Merer's original papyrus, which he logged during his trips between Tura and "Akhet-Khufu" ended up all they way over at Wadi el Jarf.
Because the crew of which Merer was in charge worked on several different consecutive operations, and brought with them records of some of those operations (12).