So, you know how Toriel, despite being a Boss Monster with the same ATK and DEF as Asgore, only has 440 HP, less than an eighth of Asgore's 3500? Well, while it is true that this was likely just to make the fight easier and shorter in-game, I like to imagine that there is a condition that can affect Monsters, one that affects and reduces the health and power of their SOULs, and thus, their stats.
I'm gonna call this "Frail SOUL Disorder." There are three main variants. The first, named FSD-V1, is the condition that Toriel has. This is the most likely and least dangerous of the variants, as it only reduces the amount of HP that a monster would normally have. However, the reduction is very drastic, reducing it to a small fraction of what it should be.
The second main variant, FSD-V2, is less common, but significantly more dangerous, as it affects all the stats of a monster, not just their HP. This is the variant that Sans has. The stat reductions are also more significant, hence why his stats are all at 1, when the HP of skeletal Monsters are usually in the late hundreds, with their ATK and DEF being around the 20s.
The third, rarest, and most dangerous variant is FSD-V3, which not only drastically reduces a Monster's stats, but also greatly affects other areas, such as their magical abilities and the amount of love, hope, and compasion that makes up their very SOUL. This, of course, puts them at a high risk of falling down from the deficiency in these necessary emotions. This condition is what some of the fallen-down Monsters who later became the Amalgamates had, though Monsters can fall down for any number of reasons, even if they have a great amount of the emotions that make up their SOUL. This variant merely increases the chances of something like that happening by a great amount.