Is the latest forecast always better than the previous one?

In principle, yes, although not always. For example, UM model forecasts starting at 00 and 12 o'clock use a wider set of initial conditions than forecasts at 06 and 18 o'clock, so they tend to be more reliable.

The second reason, more general — the model needs, especially in summer, during the period of intense convective phenomena, the so-called "run-up". This means that the first hours of the forecast may not yet show heavy rainfall or thunderstorms. It is then worth checking the forecast calculated earlier.