Spider-Man: Noir

[Minion X]

I think it is because he was not part of the experiment. He was part of the crew that was rescuing them. He got bit by the guy who had been experimented on with spider genes. No one realized he got powers and that is also why his power is stable.

There are scenes showing how his budding mutation led to involuntary movements and other issues, and a general visiting him and sugarcoating that he was now a research specimen of the US army.