Interesting. Contact is not a default company property as far as I am aware, so it may been created by someone as a custom property in your portal.
You can associate contacts with a company and vice versa from the right hand side of the contact or company record view. More info here: Manually associate contacts with companies
The create company dialog doesn't typically include a field to enter/associate a contact, so can you confirm which field you populated with the contact name?
Nonetheless however, the create company action wouldn't create a new contact. You need to associate companies with existing contacts as a second step.
Interesting. Contact is not a default company property as far as I am aware, so it may been created by someone as a custom property in your portal.
You can associate contacts with a company and vice versa from the right hand side of the contact or company record view. More info here: Manually associate contacts with companies