Why are skeletons lawful evil while zombies are neutral evil?

I’ve been creating homebrew undead monsters for my campaign and in order to do so well I’ve been dissecting existing monsters that fall within similar veins, including the low-level skeletons and zombies. It’s interesting to see the stat differences between them, considering that they can both be created from the same spell, and I can also find narrative reasons for those differences. However, one thing I can’t really figure out is the difference in alignment between the two. What makes a skeleton “lawful” while zombies are just “evil”?