Villagers born in Savannah have a chance to be born wearing plains villagers' clothes.

Maybe that villager was born on the side with the Plains biome?

Plains is the default outfit; the village generated partially outside the savanna biome into another (seemingly river or ocean), thus the villager got the default clothes.

However, most of the villages wear savannah attire, and only about 20% wear plains attire.

Yes, as the village was only PARTIALLY outside the savanna.

Even though all the houses are in the form of savannah villages, can some belong to plain villages?

the structure generation origin is in the savanna and determines the structures' looks; the moment a villager is spawned, it looks at the biome it is in, not the structure.

In the screenshot, based on the grass color, you can see that parts of the village are outside of the savanna biome.