One of the rules for nouns is that the negative examples you use (in the whole sequence, including testing) are ones the learner already knows. In this case, I think that, because there is such a narrow range of variation in letters, they felt like the already-known "d", "p", and "q" could be saved for the test examples.

I personally think it wouldn't hurt to mention them before the testing examples, too, and this seems like something open to interpretation.