I think Brickset pulls straight from Lego’s inventory.
I don’t wanna tell you you’re wrong, but… you’re wrong…
It says right there: “…obtained from Rebrickable.com.”
Ok, so for that particular set (since it’s 20 yrs old) they couldn’t pull the inventory from Lego.com, so it is being pulled from Rebrickable as a backup. Other sets that Lego does have the inventory for, are pulled from Lego.com and have an option to pull instead from Rebrickable.