Why is there a light gap around my inside-mounted blinds?

The light gap is normal physics, not a defect. To mount inside the frame and still operate freely, the blind is made narrower than the opening by the factory’s width deduction, which leaves a small clearance on each side; most shades also don’t seal to the sill. Daylight rings that clearance. It’s why an inside-mounted blackout shade never fully darkens a room. If the gap bothers you — for darkness or privacy — the answers are an outside mount that overlaps the frame, side channels that close the vertical edges, or routeless slats and a wrapped bottom rail to shrink the smaller leaks. A gap wider than the deduction on one side usually means the window is out of square, which an outside mount hides.