I've actually written to Filofax USA more than a few times pointing out the errors on their website.
So in a lazy afternoon moment I constructed a table to show how and why they get the measurements wrong:
|Fraction||Imp||mm||mm||Filofax Ring Sizes|
So as you can see there is plenty of scope for who ever fills in the details in Imperial Factions of an Inch on which ever side of the Atlantic they happen to be to get it wrong...
Lets look at the standard size Personal ring size of 23 mm, you will often see this expressed as 7/8" or 15/16" neither of which is correct you have to go to 29/32nds of an inch to get reasonably close to 23 mm. Similarly 25 mm rings are sometimes described as 15/16ths or 1 inch rather than 63/64ths... I think 1 inch is the accepted 'practice'
Because Filofax (generic) don't go below an 1/16th of an inch in their Imperial measurements you will see that they can easily over or under estimate a ring size.
I wonder when the USA will move to metric measurement.... Steve runs for cover!