Question

Suppose two scientists perform the same experiment separately, but their results are very different. What could they do to find out why this happened?

Answer 1

They should identify the confounding variable.

Some condition that is not examined by the scientist might alter the experiment result. That condition is called confounding variable. If the method of the experiment same but result is very different, there should be unidentified confounding variable. It could be air humidity, temperature, ventilation, light, time of the year or anything that might not be seen by naked eye.
Try to redo the experiment with controlling variable as much as possible.