A few of the possible answers are summarized in the following table.  Other causes are also possible.

Cause of variation Speed of effect
1.  Feed properties such as feed composition, enthalpy, and flow rate.  These could have larges effect on the distillation composition. 1.  These effects would enter at the feed tray and must influence many trays to reach the distillate product stream.  This would take many minutes.
2.  Reflux flow changes would influence the distillate composition.  This change would normally be due to the changes in the valve opening which would be known; however, changes in the pump pressure might be the cause. 2.  These changes could be faster than the effects of feed but would not be the cause of the highest frequency variation.  (see below)
3.  Sensor noise due to sampling and analysis errors. 3.  These errors would occur every measurement result, so that the noise could be very high frequency.  This might be the cause for the very fast changes in the measurement.