Why can the same emoji mean different things on different platforms?

When two platforms draw an emoji differently, the connotation can drift. The classic example is 🔫 Pistol: originally a realistic revolver, Apple switched to a bright-green water pistol in 2016, and every other major vendor eventually followed.

Another example: 😬 Grimacing Face used to look cheerful on some platforms and stressed on others, which caused real misunderstandings before designs converged.