It could be something like hobbyist vs. professional programming. I would bet that pull requests generated as part of someone's job are typically higher quality

It could be something like hobbyist vs. professional programming. I would bet that pull requests generated as part of someone's job are typically higher quality: they're more thoroughly used/tested before submission, the authors can spend more time on them, etc. These are more likely to be submitted from "work accounts", which can be deliberately vague (e.g., if shared) or just not fleshed out very well. It might be possible to flesh this out by looking at the domain names.