Quantity factors account for spawn quantity of an enemy. Items that are effective against enemies which tend to spawn in large groups get a bonus.
Threat factors account for importance or priority of a single enemy. Items that are effective against enemies which tend are high priority get a bonus. This factor ensures enemies such as bile titans are properly accounted for e.g. a 4/5 vs. bile titans is worth much more than a 4/5 vs. hunters.
The game sort of self balances this aspect of spawning a high quantity of light enemies and a low quantity of heavies however at the higher difficulties a slightly increased amount of heavies is assumed.
The actual numbers here aren't of particular importance as they are normalised after. Its the relative difference or % weight each enemy occupies which is what matters. Assumed difficulty is D8-10.