mcstiofan
A selection process is just a way for an ANSP to attempt to ensure that the people they recruit will be able to become controllers. i.e. Cost.
Generally they aim at the best of the best to attempt this. Minimum risk. i.e Cost.
You need 20 controllers, you have a course planned for 24 trainees, your recruitment system says only 12 are suitable, so you slide along the scale to fill the course. What else would you do ?
No selection process is perfect. I suspect that today's failure rates are not much better than those of 40 years ago. (where my recruitment was four wise men asking questions).
It is just a filter and there is a degree of luck and location whether you get through. Even entry standards for Universities vary globally.
It is not an ICAO requirement to have passed a recruitment test to hold an ATC licence.