Possibly the installer 10+ years ago couldn't be bothered thinking about which UHF size was required, so he just bought the biggest one.
When that aerial was installed (13 to 19 years ago) Sky installers were paid the same amount of labour irrespective of what size aerial they put up. (I was a Sky installer for 18 years). That's why I'm certain he wouldn't have bothered with the extra time and hassle of installing a 91 element aerial instead of a 23 element aerial unless he had to.
Regarding your signal being too strong, if your VHF aerial was amplified then that would have been what was overloading the system, requiring an attenuator, not the unamplified UHF signal!
Cheers, Wild Bill.