It's odd how people can relate to more than one type other than their own. I relate a good bit to INTJ and INFP along with ISFP, but only from this website, who's profiles are based on interviews. They are among the best I've ever read, and they are short and great.
I usually score as INTP/INTJ, or INFP/ISFP, and I can relate strongly to all 4 descriptions.
Can anyone help me with understanding why this happens?
Think of each parameter in terms of a scale instead of a binary value. N and S are at the opposite extremes of the N/S scale. You might be just slightly favouring one or the other instead of being near the extremes, hence the different results. Of course, how you are feeling on the particular day of the test will also affect the results.