Alien:covenant question (spoilers)

In the film David created the alien xenomorph we have all come to know in the original films on that planet while experimenting with the pathogens. 

The covenant film is set in 2107 but the original alien film is set in 2050 something so how would the eggs go back in time to the original film?

Or am I wrong or is Ridley Scott just having a time continuity error

Sherlock bonesalot -

In the film David created the alien xenomorph we have all come to know in the original films on that planet while experimenting with the pathogens. 

The covenant film is set in 2107 but the original alien film is set in 2050 something so how would the eggs go back in time to the original film?

Or am I wrong or is Ridley Scott just having a time continuity error

David created the protomorphs, the aliens in covenant. The aliens in the original movies are xenomorphs. I think, I could be getting the two mixed up. 

Alien Universe timeline from the alienanthology wiki says Alien is set in 2124. Prometheus is 2093-2094 and Covenant is 2104.