Why does the VIIIB group have 12 elements?

so I got this question that you can read above (it is about the periodic table and its groups). When I was trying to find the answer I basicaly bumped only on one page that explains it. It is important to note that this system is old one and nowdays you only number these groups from 1-18. If you want to read the answer from first hand go here: 1)

So in the new system they are not in the same group, but why were they there before?

The groups are made by comparing the physical and chemical characteristics of the top electron shells. This special VIIIB group are the collumns under Fe, Co and Ni which makes it quite unusual.

Basicly as you move across the periodic table, different oxidation states are more and more stable (in chemistry most things just want to get as stable as possible).

Those elements that have similar stable oxidation states are then put together. Take for example halogens. Those are the very very reactive elements next to noble gases.

Halogens in periodic table.

All of them will have oxidation number -I (most of the time). This puts them together. Transition metals are much more difficult to grasp with their d-orbitals. As you have seen in my other post they make some problems. The oxidation numbers for them are not very clear. Iron can be in II as well as III but generally these twelve/nine elements are occuring in the similar ones. Oxidation for these elements is actually almost always 2 or 3, this is especially true for nickel.