Why is it necessary to make a reservation that the phenomena proceed in the same way under the same initial conditions?

If the initial conditions are different, then the value of the relative values (speed, displacement, trajectory) will differ in different frames of reference.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.