How can I improve Siri's accuracy with my contacts names?

The issue you've described lacks crucial context, such as the specific application and the timing of its occurrence.

I am trying to give you a general answer.

Choose the correct language where the name is used correclty.

Add a keyboard of the desired language and if the language is supported for dictation then dictation language is automatically added under Settings > General > > Keyboard > Dictation Language

There isn't a strict language division between "Charli" and "Charlie." Both spellings are primarily used in English.

• Charlie: This is the more common spelling, traditionally used as a masculine name or a nickname for Charles.However, it's becoming increasingly popular as a unisex name.
• Charli: This is a less common variation, often seen as a more feminine or creative spelling of "Charlie." It can also be a nickname for Charlotte or Charlene.

While some sources suggest a German origin for "Charli," it's not widely recognized and both spellings are considered part of the English language.