I assume the "current weather" you get when you start the weather app on the iPhone wasn't built from a forecast, but from the data being read at weather stations a few minutes ago.
Most likely! No doubt the data is collected from the relevant stations a few minutes ago. However, the actual data was obviously not collected a few minutes ago! Do you really think that data is collected every minute?! What is more, in order for data to be collected that is relevant to every location you may decide to visit, how many stations do you think will be required? As we know, the Met Office has 45 sites throughout the UK.
Just accept the facts as they are and try not to expect too much. If you are looking for information regarding a location you are at that moment traveling to, you don't need a forcast, you need to phone the local info center and ask.