Updating not working as expected

[Jon N <jdnandroid@gmail.com>]

Hi,

I installed Jessie on a new computer about 2 months ago.  I needed
Jessie to support my network interface.  Since then I noticed that I
seeing almost no updates in Synaptic, or when using apt-get.  During
the same period on my old computer (also running Jessie) see packages
almost constantly updating.

I did look at a few things on both systems previously and didn't see
anything I could recognize as causing the difference.  Today however I
opened the preferences in Synaptic and changed the preferred
distribution from 'Always prefer highest' to 'Prefer versions from
Testing'.  Low and behold I now have a very large number of updates
waiting.

But, I don't think I should have to make that change to get normal
updates.  I'm thinking this has something to do with the repositories,
which are different between the 2 computers.  Here is my repositories
list:

deb http://security.debian.org/ jessie/updates main contrib non-free
deb-src http://security.debian.org/ jessie/updates non-free contrib main
deb http://ftp.us.debian.org/debian/ jessie main non-free contrib
deb-src http://ftp.us.debian.org/debian/ jessie main non-free contrib
deb http://ftp.us.debian.org/debian/ jessie-updates non-free contrib main
deb-src http://ftp.us.debian.org/debian/ jessie-updates non-free contrib main
deb http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free
deb-src http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free
deb http://www.deb-multimedia.org/ jessie main non-free
deb http://packages.mate-desktop.org/repo/debian/ jessie main
# deb http://linux.dropbox.com/debian/ wheezy main

My guess is the behavior I'm seeing is related to either the
'jessie-backports' or 'jessie-updates' repositories, but I don't know
why I ended up with them or why I would (or would not) want them.
Should I just change them to 'jessie'?

Thanks,
Jon