You got very good results. 3rd and 4th cousins are excellent estimates for 2nd cousin once removed.
If you review your genetics lessons you will recall that each chromosome recombines and creates segments from each of your parents before you reproduce. Which segments go into which egg or sperm is entirely random. In addition, two people can match on the same segments as a third person but with different start and end points.
Your cousins inherited different sets of segments from each of their parents, who inherited different sets of segments from each of their parents. That is why the DNA is not exact.