Startups don't have as rigidly defined team boundaries. It wouldn't be uncommon for people to take up tasks and responsibilities that would fall under some other team with a different manager.

In larger corporations, teams are insular - members aren't rewarded for doing work outside of their domain, and would be punished for letting another team do their job. Some members are so indoctrinated that they won't respond to any communication outside of their team, unless it's through their own manager.