In the German silent film Nosferatu (1922), the vampire Count Orlok bites the guy Hutter.
But then how come he alone survives while all the other people who have been bitten by that vampire die?
There doesn't appear to be any real reason for this, other than the fact it helps the story along.
Hutter doesn't know who Orlok is. When eating dinner at Orlok's mansion, he cuts his thumb and is nearly devoured by Orlok who wants his blood. He then wakes up the following morning with two puncture wounds in his neck.
Given Orlok easily kills many people later in the same way, the only logical explanation is that, for whatever reason, he wanted to keep Hutter alive - whether permanently or temporarily, we don't know.
I suspect this wasn't given too much thought when creating the movie, as it proved to be a useful plot device building the audience up to understand the terrible truth of who Count Orlok really was.