Finding a visible gap between the frame and the back of an iPhone, to the extent that a sheet of paper can slide in, is not normal and does not occur under standard usage conditions. Such an issue is most often the result of how the iPhone has been handled—for example, accidental drops, pressure, or impact that may have slightly bent or misaligned the frame.
Apple does not design its devices to naturally develop such gaps, but it does offer official accessories such as protective cases and covers that can help safeguard the iPhone. Using these accessories can significantly reduce the chances of structural damage if the device is dropped accidentally, keeping the frame and back panel properly aligned.