The character will be probably as confused as you, unless they spend a good while to think over the meaning of the greeting. But just in case they paused to think it over...
A greeting is wishing the upcoming time to be good. If you say "Good morning" you don't state "this morning is good". It's a shorthand for "I wish you to have a good morning".
Since at noon the morning is just ending, there is no point giving wishes for such a short time. You wish good afternoongood afternoon which has just started - your wish will last for longer.