Why did ubuntu install break out my partitions?

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http://askubuntu.com – I have a 750GB hard disk with Windows 7 installed. So I shrunk the C: drive to about 80 GB, created two more partitions, one for data (about 615GB), one for ubuntu (30GB) (extended partition). I ran gparted, created a 8.5 GB swap partition inside ubuntu partition, and left whatever to ubuntu root partition. So I went to install ubuntu. After a few tries, everything worked. But now, I noticed some puzzling facts. Here are the output of a few commands: $ df Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda7 5333936 4472688 590300 89% / udev (HowTos)