The same words or labels are being used in both systems, which gives the impression that they are the same. They misunderstand each other - and because they assume that the other person has understood the term in the same way as themselves, they may not realise that that they have misunderstood each other. The same term is used in the two different languages, to mean different things. Whilst there may be correlation between temperament and type, my contention is that they are not equivalent. In Please Understand Me, Keirsey makes the association between Jungian typology and temperament.
Jungian typology has to be changed in some way to make it fit with temperament.
Unfortunately, because the same letter is used in both systems, the misunderstanding may not be recognised. Another criticism from the above quotation is that Jung confused Sensation with Extraversion. In fact, this is not true. Jung's description of the thought processes he went through before arriving at his final theory. It is clear, from Psychological Types, that in Jung's final theory, Sensing and Intuition had been completely separated from Extraversion and Introversion, and there was no confusion in his own mind.