Because the underlying 90 miles in 30 minutes was originally based on an efficiency assumption.Why would that matter? It's not a percentage in an amount of time...it's amount of miles during a time period. On top of that the wording is "GM estimates" of "up to" 90 miles, in "about" 30 min....whatever the heck that means.
Another GM claim from January was 80% charge in 60 minutes which you sometime see in the form of 160 miles in 60 minutes which is obviously based on an assumed 200 mile range.
If the final EPA combined range is 238 then 80% of 238 is 190.4 miles in 60 minutes.