ᐅ Why do the shelves of the BILLY unit sometimes not fit perfectly?

Hello everyone, I have encountered an issue with the shelves of the Billy bookcase from IKEA. I noticed that the shelves do not fit precisely into the designated holes on some units. In some cases, they fit very tightly, in others rather loosely, or they have to be pushed in forcefully, which is uncomfortable and can damage the wood. I am wondering whether this is due to production inaccuracies, manufacturing tolerances, or perhaps material warping caused by moisture. Does anyone know the exact causes or have suggestions on how to install the shelves more easily without damaging them? Thanks in advance for any informed answers!