This is not a mistake this is a new visa rule recently enforced. If you enter Thailand on a {VOA} which is a 30 day free visa on arrival and you wish to go on a Cambodian visa run or cross a border via land, you will now only receive 15 days when you re-enter. If you leave Thailand via the airport and re-enter using the airport and not across land, you shall be given 30 days.
This rule does not apply if you have a current visa in your passport which was acquired from outside Thailand e.g. Non-Immigrant visa multiple entry. Please note: Not all nationalities are given 30 days on arrival so please check your visa status before you arrive. New enforced rules can be less of a headache if you arrive in Thailand with a visa type in your passport.