No systematic empirical study on investigating the effects of performing task variants on user cognitive strategy and behaviour in usability tests and on learnability of the system being tested has been documented in the literature. The current use-inspired basic research work aims to identify the underlying cognitive mechanisms and the practical implications of this specific endeavour. The focus of our work was to assess user rationality and system learnability. The software application tested was a multilingual learning resource repository. Eleven German and eleven Slovenian participants were involved in two user tests (UTs). Usability problems (UPs) identified in two quasi-isomorphic tasks were categorized with respect to a scheme of associated skills. Actions of the two tasks of each of the 22 users were segmented and coded according to a scheme of cognitive activities. Results showed that generally the users adopted different strategies for working out the given task and its variant, and that the system could be proved learnable. User Rational Action Model and implications for future research on user tests are inferred. (Contains 8 tables, 4 figures and 4 footnotes.)