I think the 'assumed' playstyle is that you might be running around very hurriedly during an adventure, but afterwards you'll spend quite a while resting- so you gain two levels during the adventure, and then spend two months crafting some new gear, updating your spellbook, and writing down some scrolls (admittedly this is largely for the benefit of your spellcasters; other classes don't need anywhere near that much downtime and usually don't get much mechanical benefit for having it.)