Due to a certain range of deviation between the pre-drilled holes and the assembly progress, it might occur that the pre-drilled holes on the desktop don't match with the holes on the crossbar.
In that case, you just need to ensure that both side brackets already well attached to the desktop. Then you can simply drill new holes directly on the desktop by yourself for the ones in the middle of the crossbar. Please rest assured that this action will not at all affect the function of your desktop.