Actually, you may be trying to apply a theory (quantum mechanics() that works at the microscopic level to the macroscopic one. Observations are indeed acting on the nature of the system.

Although here it is valid at the microscopic level (the hard process of the collision obeys to the laws of the quantum world), when we are far enough from the collision point (where the detectors are), those effects are not there anymore.

However, this is true that the results of each collision will be different. As I said before, this is why we need to repeat it a lot :)