Why is weather forecasting in the UK so bad?

The Met Office weather app screen

On my iPod Touch 4g I have the Met Office weather forecasting app. It is great with forecasts every 3 hours for the current day and for the following few days. There is a lot of detail there. The problem is it is frequently wrong, even at the current time!

I know all the excuses about butterfly wings flapping, but with huge mega-computers now in use is it really so impossible to get a decent weather forecast just a few hours in the future?