Siwel, my Nan always said it was because grooms were expected to be older than brides, so the retirement age difference allowed them to retire together!
I can’t find any good resources for an alternative viewpoint, but it seems that a 5 year age difference occurs pretty much world over.
Anyway, I have no issue with equalising the retirement age, the only problem is that a certain group of women born in the 1950s have had this change foisted on them without enough warning to plan for it.