The point here is standardisation. I did my training some time ago in a different country, but I'm sure some issues are the same. To achieve a standard training delivery you need the following:
Standard procedures - On my course we had instructors from enroute, approach & towers. We had ambitious, keen, young guys who had just made full-performance level and crusty old pr1cks who had not moved from behind a desk for decades. These people used to argue endlessly among themselves over basic procedures, and carp at trainees for doing exactly what their last instructor had TOLD them to do. Once we had a list of new expectations read out to us in the corridor while waiting to enter the simulator for an exam...
Standard approach - Some instructors would scent blood in the water. If a trainee seemed a bit uncertain they could be hounded by hypotheticals and "what have you got between those two" in the hope of pushing them over the edge. Somebody who seemed to be cruising could have his trainer sit back making yummy noises. Some just hated women and tried their best to unsettle them. During one exam that must have been boring for him (wasn't for me) my instructor placed his leg on the console next to me and let go a fart that sounded like a tearing phone book. "Don't worry boys, it'll smell of crushed rose petals..."
Standard environment - Some blip-drivers are retired or aspiring pilots or controllers, some have buggar all experience. Some are keen and want to provide the most real environment they can for the trainees, others are watching the clock or deriving amusment from confounding them. During another exam one of my blippies had left a line open. After determining it wasn't me I had my mouth open to tell them when I hear, "Ready? 1, 2, 3, go!". Immediately an aircraft calls while two more hot-lines open and the blippies all start talking over each other until I sort it out. And it kept happening. And it didn't happen to the guy sitting next to me.