There may be two reasons for this.

Do not be lazy and take a roulette with you in order to measure the diagonal of the door. If you identify the discrepancy by 1 millimeter or more, it is better to take a closer look at another copy. There may be two reasons for this. Firstly, this is the use of obsolete equipment and technologies in the factory, and secondly, deformation of wood as a result of the academic of the material. If all of the above defects were not found, then it is worth thinking about buying euros doors with measurement and installing.