Ubuntu is Debian based
I'm not sure if this helps, but it should give some direction. From past experience with Net-Bsd, I found the same thing. It would have a root partition on hda1, then skip three and have the swap on hda5. I looked into this a little and found that Unix, not Linux, uses three extended partitions for various reasons. I'm not too sure but I think one of them may be a floppy partition, or a cdrom boot partition. If someone know more please feel free to correct me. Anyway, I think that Debian is closely based off of the actual Unix, and that's why it partitions like that. If this doesn't answer your question, and no-one else posts. You could easily get an answer from the distribution section, under Debian. But I hope this sheds some light on it.