Inaya and Vihaan are reading where the end of an eraser lies on a ruler

Why

Vihaan is correct that the marks on the ruler never move. But a reading isn't made by the marks alone — it is made by lining up the edge of the eraser with a mark, as seen by your eye. A ruler has some thickness, so its marks sit a little below or beside the eraser's edge. Seen from the side, the edge appears to line up with a different mark from the one it really sits against; move your head left and right and the reading seems to shift. Only when the eye is exactly above the point being read does the edge line up with the correct mark. Test: hold a pencil upright beside a ruler and read its position with your eye first to the left, then to the right, then straight above — the first two readings disagree, and only the third is correct. Inaya is right: the mark must be read from directly above, because reading from an angle shifts the reading.