Don’t worry - and no need to be confused about it, as there is a reason:
Indeed there are newer versions. They are called “pre-releases”.
You can always find the latest pre-release here:
https://github.com/triplea-game/triplea/releases
As of writing this, version 1.9.0.0.7422 is the newest one.
The file you want to download for the Mac then is this one: TripleA_1.9.0.0.7422_macos.dmg
From time to time the TripleA devs decide to mark a new (pre-)release as “stable” and offer it on the www.triplea-game.org website - but development is so fast, you sometimes have five or more pre-releases a day.
You can have more than one installation of TripleA side by side. So you might want to give the latest pre-release a try.