Something described as "fundamental" typically means we're at the peak of our understanding. It often doesn't actually mean it's purely fundamental, but we can't figure out anything beyond it.

e.g. one of the most perplexing things in physics is the occurrence of the fine-structure constant. Roughly 1/137. We have never found why this specific ratio is so prominent in physics. But it exists.

Now imagine a scenario where you were some leading philosopher in ~2000BC doing some experiments and kept finding the number 9.86 over and over and over and over with no known reason explaining why. Later generations of human beings came to realize that what you found was just π^(2). Your generation of human beings just didn't understand the geometry of circles yet.

Chances are we don't understand the structure of the universe that's emitting this 1/137 and that there is some more fundamental rule describing it, but as of yet we acknowledge it as "fundamental" in admittance of our ignorance.