When using the given crisp functional relationships in a fuzzy environment with fuzzy number input, always the problem arises how to specify the fuzzy numbers and to choose the t-norm within the extension principle. In this paper the question is considered, whether different pairs of reference function and t-norm can lead to the same fuzzy value for the relationship. The question is answered in the negative. However, for more general combination functions inserted into the extension principle this case will happen as a rule.