I can't understand why you'd do a gravity check before the end of the boil. At that point, you have no real target to compare to. It would be perfectly normal for the gravity to be lower than normal at that point, because there will be additional boil off before flame out, that will further concentrate the wort.
Gravity is dependent on volume. With extract, if the volume is correct the gravity will also be correct within a very few points. The gravity contribution of steeped grains isn't generally enough to make much of a difference so, as long as you measured everything correctly at the beginning and had the expected volume at the end, your gravity will be close enough that there's no real benefit in making post boil. adjustments.