Weapon Chest drop - Weapons are obtainable from Weapon Chests in the area listed and all areas after. Weapon drops are calculated by the difficulty number. Difficulty is the number of portals you entered; it determines the amount of enemies spawning in the area. Entering a Crown Vault or the Sewers Secret Area counts as entering a portal and increases the difficulty by one each time you exit them, so you get better weapons sooner but there are more enemies.
If a weapon is meant to drop on difficulty 11 that would mean the weapon drops in area 5-3 from enemies, 5-2 from Weapon Chests, and 4-1 from Cursed Chests. Robot's passive reduces the number by one (5-2, 5-1 and 3-3).