The reason is to have a text that can be understood and that is exact, its not just me who misunderstood the current text.
Current text in wiki:
"+10% to sum of Yields,

Golden Age duration modifiers, and Combat Modifiers from Global
Monopolies; +3 to flat Yields and

Happiness from Global Monopolies."
+10% to sum yields, what yields? doesnt say anything about what/where, there are no global combat modifiers, it doesnt say that happiness only apply to one monopoly.
This description is just bad.
However you're right regarding combat modifiers which then isnt needed at all since there are no global combat modifiers from monopolys, ie that part could just be removed from the description entirely.
Global Monopoly bonuses: +10% to every unique monopoly yield, +10% for at least one Golden Age duration modifier, +3 to every unique tile monopoly yield and +3 happiness for one happiness monopoly.