APT ordering of reposiitories

[Bill Purvis]

I'm trying to maintain two machines in parallel, but find that apt, 
despite having what
appears to have the same list of repositories has installed different 
versions of some packages
on the two machines.

For instance: iproute2. This is on one machine as:
4.18.0-1ubuntu2~ubuntu18
while on the other, it is
4.15.0-2ubuntu1

Running 'apt upgrade iproute2' on each machine states that this it the
latest current version.

Doing a bit of digging in /var/lib/apt/lists, I find that iproute2 
appears in both

archive.ubuntu.com_ubuntu_dists_bionic_main_binary-amd64_Packages
and
archive.ubuntu.com_ubuntu_dists_bionic-backports_main_binary-amd64_Packages

with the -backports version being the 4.18.0 version.

Is there a reason why one machine is ignoring the later version?
How can I force them to match up?

For background information:
I'm actually running Linux Mint 19.3 here, and the two machines got there by
different routes - the one with the latest version was an upgrade from 18.3
whereas the other was installed from the Mint 19 image. Both came up as 18.1
and then I ran apt dist-upgrade to get to 19.3.

Many thanks

Bill
(who has run Linux since it came on 40+ floppy disks....)

-- 
+----------------------------------------+
| Bill Purvis                            |
| email: bill at billp.org                  |
+----------------------------------------+