cannot install from backports

Greetings!

As the title says, I can't seem to install anything from backports.

Example, lets say I need to upgrade Gimp from 2.8 (in stable) to 2.10 (in Testing).If I go to https://backports.debian.org/Packages/ I can verify that 2.10 is indeed in testing.

I run this:

apt -t stretch-backports install gimp

And my newly installed Debian 9.9 replies:

gimp is already the newest version (2.8.18-1+deb9u1).
0 upgraded, 0 newly installed, 0 to remove and 153 not upgraded.

Which is of course WRONG, because we just looked in Packages and verified that 2.10 is in testing.

What can I do to install Gimp 2.10 from testing?

Thanks in advance,

--Brian

Also I have run--as root--apt update, and here is my /etc/apt/sources.list file:

#copied all this from debian https://wiki.debian.org/SourcesList
deb http://deb.debian.org/debian stretch main contrib non-free
deb-src http://deb.debian.org/debian stretch main contrib non-free

deb http://deb.debian.org/debian-security/ stretch/updates main contrib non-free
deb-src http://deb.debian.org/debian-security/ stretch/updates main contrib non-free

deb http://deb.debian.org/debian stretch-updates main contrib non-free
deb-src http://deb.debian.org/debian stretch-updates main contrib non-free

#backports added per debian here
deb http://deb.debian.org/debian stretch-backports main contrib non-free
deb-src http://deb.debian.org/debian stretch-backports main contrib non-free
deb http://security.debian.org/ stretch/updates contrib non-free main

--Brian

Fantastic Fantasy, Curiously Chronicled

Links for all the major platforms -> http://BrianCaryBooks.com