It is a calculated value, with a certain degree of uncertainty of course, but your number is way off.
I used 8,000 km/year for EV average milage or 414 miles/month.
Bear with me for the mix of units, but there is a good reason for that.
Using the autolife registration numbers, each vehicle then gets full milage for the month of registrations.
An EV registered in January 2022 have done (28 month + 16.5 days) x 414 miles.
A February 2022 registrations gets 27 montsh and so forth.
Vehicles registered in 2020 and 2021 starts from mid year, as I don't have monthly registrations data.
Vehicles registered prior to 2020 are not included, again because I don't have the data.