Not The Same San Francisco Bay

The image shows San Francisco Bay, FirstSide. On the Other Side, Tom Christiansen stares open-mouthed at the Bay with:

no San Francisco;
a town along the waterfront;
a small airport in place of the Naval Air Station;
no landfills on the shore;
long piers with fishing trawlers and small wooden ships;
cranes taking cargo to sheds and warehouses;
trucks carrying it to the highway;
no bridges;
Alcatraz with only swarming pelicans and guano;
fewer and smaller ships - schooners, barges, tugs, sailboats and fishing boats;
in flight, gulls, pelicans, cormorants, golden eagles and several condors;
a spouting whale;
sea lions;
a sea otter.

Adrienne explains:

"'...it's exactly what it looks like, only it's not the same San Francisco Bay you grew up with. Different...time line. A different history, two different universes existing in the same space and joined only by the Gate.'" You know the concept?'" (p. 243)

In Narnia, they would say that you cannot go there by traveling through space, only by magic.

Tom replies:

"'Yeah, you know I read science fiction.'" (ibid.)

And we know that that science fiction includes "Anderson" (p. 103) so Tom has almost certainly read Valeria's explanation to Rupert:

"'You can picture the cosmoses as lying parallel to each other, like the leaves in a book. That isn't strictly true, either; they occupy the same space-time, being separated by a set of dimensions -'"
-Poul Anderson, A Midsummer Tempest (London, 1975), p. 101.