The closest related viruses to the mexican swine flu are viruses isolated in labs in research facilities. If it were a completely “naturally occuring” virus, the lab strains and the one we are seeing would be significantly different.
Think of it as the more time the virus is in the “wild” the more mutated it becomes, therefore it doesn’t make sense that it is so similar to lab strains.
Essentially, someone took sequences from the H5N1 strain and H3N2 and others and combined them to create this virus.