Probably that there's an underlying common cause that means the effect disappears when you analyze the data properly. For example, if screen time is caused by l

Probably that there's an underlying common cause that means the effect disappears when you analyze the data properly. For example, if screen time is caused by lack of parental presence and lack of parental presence is caused by poverty and poverty also causes depression, then a naive analysis would find a correlation but a proper analysis or meta-analysis would subtract out the common causes of poverty and parental absence and find no effect.