(and contrary to what I've heard, the Electro scenes aren't as tacked-on as some assert - there *is* an arc and growth to the character)
The recently-released sequel film includes an aspect of biology few people think much about…(or think about much)
Peter’s dad used his own DNA so Oscorp couldn’t continue the research on the spiders. (uh, DNA doesn’t care who is manhandling it, doc)
…but the hybrid aspect was strongest in the venom, which is what housed the regenerative compounds.
Now, as far as I could tell, the spiders which bit Peter (last film) and the ones which donated to the venom collection (this film) were two populations, or that was the impression I got.
Which means: genetic drift.
If a feature or gene is neither harmful nor helpful to survival and propigation, it is considered neutral. Mutations tend to collect in neutral genes. Which meant it probably was something as simple as genetic variation, solidified through founder effect/bottlenecks in the establishing of two spider populations…which created the Green Goblin.