Why is the statement “all life is made of cells” ab example of a theroy? explain.

Theory-is an explanation of something, based on a body of facts that have been confirmed multiple times through experiment. The statement “All life is made of cells” has been confirmed multiple times through experiment but this is theory becasue there is still a "chance" that it could be wrong. To better explain this, if i were to test wheter gravity exsist on earth, I would drop a pen and the pen would it the  ground. I could do this 1 billion times but there is a psosblity that the 1.1 billion time the pen could not drop to the ground. Basically, its  never been disproven other wise that “All life is made of cells”.