Ubuntu's distro team is a very well-oiled machine. The same isn't true of development projects involving more than one or two people. There's a good reason for that: Distro started with a team of people who already had all the needed skills and had worked together, and retained that culture even as people changed. They produced the first release in six months, so they knew they could do it again and were careful about who they brought on while they built the infrastructure to keep it going. The development side, however, has typically not been staffed with program/project managers but instead by developers promoted into management roles.

It's been a long time since I was at Canonical, so I can't speak to their current makeup, but I'd be very surprised if the schedule given was one vetted by experienced project planners given the complete lack of wiggle room.