Why did Ada in the Resident Evil series come back after dying in Resident Evil 2?

The deaths just don't make any sense. Are there any explanations for the bizarre events? (Ada died in two different cutscenes, both different ends, and returned in RE6!) (Steve died in Code Veronica but returned in the Dark side Chronicles!)

Solution 1:

Steve died in RE Code Veronica X for sure, but The dark side chronicles is more like a remake or a spin-off of the game; same bosses, same places, etc.

Ada did not die in RE2, as far as I remember after she gets hit I think from William Birkin in that trolley, Leon helps her to rest and goes out to kill one of the most irritating RE bosses ever

P.S.: In Resident Evil Code Veronica X ending, Wesker said "Maybe he'll come back alive just as I did, and to see your sister again". I really wish this become true!