When does ratiometric measurement account for signal variations?
National Instruments$^{TM}$ gives a good explanation here, in a paper titled "Measuring Bridge-Based Sensors with the Ratiometric Approach, Publish Date: Feb 11, 2016".  Here are a couple excerpts relevant to your question:  


  1) "The fact that the mV/V ratio stays constant for changes in the excitation voltage increases measurement stability. Small changes in the environment due to temperature differences typically affect excitation voltages. Because the excitation voltage is sensed and fed back to the ADC, any changes play a smaller role in the accuracy and stability of the measurement."  
  
  2) "By using the excitation as the source for the ADC reference, the output of the ADC provides measurements that are in units of V/V. This reduces the amount of scaling required to return the appropriate units."  


So, the short of it is that noise, particularly that arising from power fluctuations, in either the source or detector in the measurement systems as you described is largely cancelled out when ratioed to a reference system.   

This approach is most effective when you are rapidly switching, i.e. via a chopper mirror, between the sample measurement source and detector and that of your reference or blank measurement.  When the signal from the source is then ratioed to that of the reference system, then any noise common to the two is effectively cancelled out, increasing your signal to noise ratio.  This method addresses your examples of "sequential" measurements. In the system describe above, you are effectively taking your reference measurement simultaneously or in parallel rather than sequentially.