It is because, you might be having multiple languages in your site but have not created the email template for all the languages. For example: You are having following such languages in your site (English, German) and created the email templates only for the language English. If customer places an order using the language German, then he will receive the default email because there is no email template for the language German.
So always you should make sure that you have created email template for all your site's languages.
The following screenshots could help you understand better:
Languages installed on the site:
Language for which the templates are created:
The templates are created only for English(US). Now this being the case, when users place order using the Deutsch language or the French language, the default email will be sent, since there are no email templates exist for Deutsch or French.