Why does my USPS express mail take 2 days to some zip codes, when the same zip code sending to me take 1 day?

In my town, at least, it has to do with the time the mail leaves the main post office, not some branch office. If it is in and ready to go before noon, 1 day. If is in and ready to go after noon, 2 days.|||Weather, traffic, it gets thrown in the wrong bin, postal employee error, many different factors. Nothing is perfect, especially the mail.