According to
James Gunn, on this movie "we see the difference between Superman, who was sent to Earth and raised by loving parents from the time he's an infant, versus Supergirl, who was raised on a rock chip off of Krypton, and watched everyone around her die and be killed in terrible ways for the first 14 years of her life". Gunn indicates that this incarnation is an Argo City survivor rather than being from Krypton itself, more akin to her comic book debut in 1959.
