They acknowledge that tasks that attempt to test a certain executive function such as memory have some variability amongst them, for this reason they used factor analysis
I know the sentence contains some jargon, so I will try to explain the logic as best as I can. The variability among the tasks is why they use factor analysis, however it most certainly is not the only thing they could have done. Is there a difference between therefore and for this reason in such a situation. My intuition tells me that therefore implies more certainty that this is the only possible effect.