Did you know not all rulers are created equally?
This could very much be due to a calibration error, where the scales of certain rulers are not printed correctly.
Please check your ruler against other rulers (likely only 1 out of several rulers are out of order) or use this website: https://www.ginifab.com/feeds/cm_to_inch/actual_size_ruler.html
Having a misaligned ruler can lead to inaccuracies in problems dealing with scales and proportionalities.
All the best for your upcoming examinations!