I guess genetics are at play. For example, I have a genetic mutation that means I metabolise tricyclics much faster (and tricyclics block the reabsorption of serotonin and norepinephrine, much like SNRIs). Rate of metabolism would affect peak plasma concentrations and half life.
It's quite possible that similar mutations exist that affect SSRI metabolism.
It's quite possible that similar mutations exist that affect SSRI metabolism.