Debian Stretch was released June 2017 and Ubuntu 18.04, well, in April 2018, so its not too surprising that they ship with different Python versions.
I do find it surprising that Linux distributions don’t always choose upstream LTS versions for their releases, it just feels like a big waste of effort to backport fixes across many versions.
Actually, Debian Stretch was released in April 2015, and their general freeze was set at November 2016.
Debian spends about 4 years producing a stable release, which excels at stability at the expense of using software releases that are far from the bleeding edge.
