 Solution

A context-free grammar is an ambiguous grammar if some word in its generated formal language has two distinct parse trees. Equivalently, that word has two distinct leftmost derivations from the start nonterminal symbol. A leftmost derivation always expands the leftmost remaining nonterminal symbol.

Two arbitrary grammar derivations of the same word do not by themselves prove ambiguity: expanding independent nonterminal symbols in a different order can describe the same parse tree. Requiring distinct parse trees, or distinct leftmost derivations, avoids that issue.