The discrepencies could be due to copyist errors; St. Luke’s original inerrant Acts is not extant, only handwritten copies (or rather copies of copies of copies, etc.) made by fallible copyists.
In A Commentary On The New Testament, prepared by the Catholic Biblical Association in 1942, on page 380, it says:
Some of Stephen’s statements differ from the accounts in the Old Testament. Abraham’s call (2) came in Haran according to Genesis 12:1; Joseph’s kindred (14) numbered only seventy according to Genesis 46:27; the tomb bought by Abraham (16) was at Mambre (Hebron) according to Genesis 50:13. In these and similar instances Stephen was following the readings in the Septuagint, the Greek translation of the Hebrew Old Testament, or current traditions, or he was summarizing from memory. As an inspired writer St. Luke gives a faithful record of what was said without guaranteeing the accuracy of the statements themselves.
