Yes high submissions with no or low involvement in the community. Either they really, really, really, really love submission karma or they're here to exploit the site.
It should be pretty hard to automate spam comments (without the comments getting downvoted to hell). Whereas automated submission of stories is super trivial.
Average votes/submission seems a better gauge than raw number of submissions. Maybe there are people who are just really good an finding and posting a lot of stuff that's interesting.