It isn't.
Per STO Lore the Ross is a new design inspired by the Galaxy but using 2380s technology, but none of them are refits of existing Galaxy classes.
That's quite odd if you ask me and a bit of a waste of resources when a design very similar to a pre-existing one appears but it turns out to be a completely new class of ships.
The Galaxy class in the 2380-ies would likely be using technology of that era too... some may be a few years behind (depending on when they're next scheduled to go into spacedock or if the upgrades can be done by the crews on the go - which they technically should be able).
If the Ross was inspired by the Galaxy class, then why not just use existing Galaxy class ships to refit them?
Its a pre-existing hull, the raw material is there... it would be fairly simple to do with replicators, transporters and tractor beams.
Same with that Excelsior look-alike we saw (the Obena I think it was called?) on Lower Decks in season 2 (the one Sonya Gomez was a captain of).
While its overall shape is more oval and sports Sovereign class nacelles, bottom line is there was no need to make a completely new class of ships that looks almost identical to the pre-existing Excelsior class ships.
Would have been easier to just say it was a 24th century refit of existing Excelsiors in service rather than a new class (I mean, the thing looks nearly identical to an Excelsior as is... would have been easier to just use replicators and transporters to disassemble the saucer into base elements and replicate new oval one in its place. Same with the interiors and nacelles).
Some extra energy might be needed (depending on the resource requirements), but it still beats making a new ship from scratch.