The difference is rather simple - the total family of Jacob was 70 people.
Joseph had two children + Jacob himself were obviously four people. Therefore, we have:
(a) 66 people other than Joseph's family and Jacob + (b) four people of Joseph's family + Jacob = 70 people in total.
Note the difference in the carefully worded sentences:
(a) all the people who came to Egypt belonging to Jacob does NOT include Joseph's family who were already there & Jacob himself. Ellicott expresses it this way:
(26) All the souls were threescore and six.—This total is obtained by
omitting Jacob, Joseph, and Joseph’s two sons. If we include these,
the whole number becomes threescore and ten, as in Genesis 46:27.
Similarly for Barnes:
All the souls that went with Jacob into Egypt, "that came out of his
loins," were eleven sons, one daughter, fifty grandchildren, and four
great-grandsons; in all, sixty-six. Jacob, Joseph and his two sons,
are four; and thus, all the souls belonging to the family of Jacob
which went into Egypt were seventy.