Here's why:
* Homographs are words that are spelled the same but have different meanings.
* Homophones are words that sound the same but have different meanings and spellings.
"Plains" can refer to:
* A large, flat area of land with few trees (e.g., the Great Plains)
* A simple, unadorned surface (e.g., a plain wooden table)
While "plains" has multiple meanings, it's always spelled the same. Therefore, it's a homograph.