You're right thats its not actually every time but neither is this Edge install thing. If you click "Later" I think it won't do it for 3 days according to this blog post [0] and I think that's still accurate. If you hit Try Now I think you get a little more time to whenever Apple updates Safari/macOS.
[0] https://www.ctrl.blog/entry/how-to-osx-try-safari-promotion....