System Update vs. Software Installer speed difference?

[brainpicker]

Has anyone else had this problem? When I update any laptop using System Update the downloads are painfully slow, but then using Software Installer to obtain the exact same files clocks in at nearly 20x faster using the same connection. Do they use different servers to operate? Over and over, day after day, I get the same result over the past few months. I wonder if maybe I'm not doing something right or if others have noticed this.

Thanks,
- Yak