If all students start with the same amount of 10,000 x g supernatant, and after different groups of students have done their 40-65% ammonium sulfate cut, their results tend to vary greatly. What are likely reasons for this variance?

The question relates to a crude sample of enzyme that is in the process of becoming purified. The ammonium sulfate is being added to pull extra molecules off the proteins so that they will aggregate together and come out of the solution. In this case, it looks as if they want to give consideration to how saturation plays a part in how much of the supernatant a person will get and how/why variation occurs between the amount that each person will get based on how much ammonium sulfate is added into the solution.

I was thinking that if everyone started out with the same amount of supernatant and if the ammonium sulfate needed to be measured out based on the volume of supernatant present, maybe it's possible that different amounts of ammonium sulfate could have been measured out in the beginning before it was added to the supernatant. Otherwise maybe that it's possible that the rate of speed at which the amount of ammonium sulfate added to the supernatant may have been different which could impact the rate at which the supernatant becomes saturated? Any other input on this would be great.