Abram (later Abraham) is told his seed will leave Egypt after 400 years (see Gen. 15:13). Yet, here we read it was 430 years, which is also mentioned in the New Testament (see Acts 7:6). Why is there a thirty-year difference in these two accounts? This list may explain the difference:
Abram (Abraham) is seventy-five years of age when God tells him to depart for the Promised Land (see Gen. 12:4).
Abraham is one hundred years old when the promised seed, Isaac, is born (see Gen. 21:5). The time from age seventy-five to age one hundred is twenty-five years.
Abraham eventually removes Ishmael (at about age fifteen) when Isaac is weaned (see Gen. 21:8-21).
If the prophetic count starts from the giving of the “promise” to Abram when he is seventy-five and continues to the Exodus, the total time is about 430 years. If the count begins with Isaac is being weaned, it becomes about 400 years until the time of Israel’s departure from Egypt.
From Page 145 of the Perry Stone Hebraic Prophetic Old Testament Study Bible