Why did Lee and Keiko assume Godzilla survived the nuke?

Lee and Keiko encountered Godzilla again on Monster Island, after having assumed he'd been killed during Castle Bravo. They both immediately decided it was the same Godzilla, having survived the nuke.

Obviously from our perspective we know it to be the same Godzilla, but from theirs... wouldn't it make much more sense to assume there are simply multiple individuals in the species, and that they were seeing another?