This form is not official and should only be used as a very basic prediction of a degree classification.
Univerisites and other insitiaitons use a vareity of techniques when calculating degree grades, which may include discounting certain modules or dropping the lowest modules, and so on. This means that no universal degree calculator or predictor such as this will ever be accurate.
Consult your University handbook - and even your departments gudieliens, which may differ even still from the rest of the university - to know how your institiation calculates degree classification.
I agree