Sorry to jump in here, but I'm actually picking up a XC90 T8 tomorrow. And, like Zaxxon, expecting to get a M3 later.
That 59 number is actually the "eMPG" rating that was published all over the place before they had actual efficiency ratings available. So from what I can tell, it has no actual real bearing on range and mileage (unless you like thinking of electric motor efficiency in terms of gallons of gas, which I find weird).
The actual published rating is 24 mpg city, and 27 mpg highway.
So imagine a all-at-once "city" drive (with no charging) would be more like apprx 15-20 miles on battery, plus another 300 or so in gas/hybrid. Although I can't imagine driving for 320+ miles straight in the city
59 is the eMPG rating for the all electric mode may most likely be the case. I was thinking it was for a Hybrid drive with a fully charged pack. If you read this article, the author seems to have got > 35 miles/gal for his driving over a week which is well over the 24 mpg rated.
Volvo Beats Tesla At Its Own Game With The New XC90 T8 Plug-in Hybrid (Review)
Because the actual mileage may vary based on driving conditions, landscape etc, they may have chose to just state gas only and motor only range and the hybrid range falls between those two.