Difference between monster level and dungeon level. If you are on dungeon level 1 and get a random encounter for a monster that's level 2, you roll 1d6 for number appearing and divide by 2, so you get between 1 and 3 monsters.
Conversely if you are on dungeon level 2 and get a random encounter for a monster that's level 1, you roll 1d6 for number appearing and multiply by 2. Then you get between 2 and 12 monsters.