In searching online, I've noticed there are a lot of different ways to group the elements of the periodic table.
Take mercury in the two tables linked below, for example:
On the first table, it's explicitly outside of the transition metals. On the second table, it's included in the "transition metals" group.
Mercury is an example of this inconsistent classification, but in general, why is there no canonical grouping for all elements, across the periodic table? I see some tables mentioning "metalloids," but others not doing so. Or, sometimes elements are classified in entirely different groups altogether (like zinc, cadmium and mercury in the above-linked tables). Or, the lanthanides and actinides being included sometimes in the transition metals group, and sometimes not.
So which classification is right? Or rather, which one is the "most right"? Really: which one should I focus my efforts on remembering?