I think another source of variation on the results is the actual nutritional intake of the subjects.
The vegan/vegetarian is a big spectrum. A vegetarian eating plenty of dairy products and eggs should in theory be much better creatine-equipped than a poor vegan student feeding on noodles and ketchup pastas. A lot of pseudo-meat for vegans are full of processed shit.
Confusion will be even higher if you include fish in the vegetarian diet, which is odd but some people do that.
Could the cognitive effect works in the case of creatine deficiency (vegan) and not in the case a vegetarian (low creatine but not deficient) ?