Actually it IS consistent, and is scientifically based. The Fire Danger Ratings are based on a calculated Fire Danger Index, which takes into account forecast temperature, humidity & wind speed + how dry the environment is (called "Drought Factor") . I won't go into the science behind it - if you're interested google McArthur Fire Danger Index.
It's actually a measure of how difficult a fire will be to put out / how it might behave IF IT STARTS. It's not a measure of how likely fires are to occur (over 75% of fires are caused by people - you can't predict where or when that will happen !)
Where it can sometimes look strange, is the published Fire Danger Rating is based on the highest
for a given weather forecast district. As an example, if the East Gippsland forecast district has say 15% of it's area forecast to be "Very High", 70% "High" and the rest "Moderate", then the rating will be "Very High". Usually it's based on the highest 10%. There is both a Forest FDI and Grassland FDI calculated - different Districts use one or the other (For example the Mallee & Wimmera are based on ther Grassland one, Gippsland on the Forest one.
"Code Red" days are actually DECLARED rather than calculated - eg. when the calculated FDI is off the scale. Similarly, Total Fire Bans are declared by the CFA (in Victoria and are designed to prevent fires starting / being lit. They don't come in a a standardized FDI.
Hopefully that helps
Yes, I do this for a living .....