light detection system: Response different from power bank and computer system
Based on limited knowledge of the design and limited knowledge of the requirements here are 3 possible reasons.  More may exist.


The USB 2.0 definition stats such a port provide 0.5A of current.  Some devices limit the current as a precaution.  Therefor the battery may provide more current then the USB source.  The type of USB source was not specified and no voltage measurement data was provided.
How fast does the device need to react to a change in light level?  A light-dependent resistor or LDR can have a latency of as much as a second with respect to light level changes.
With regards to the 16 bit ADC.  What precautions have been made with regards to precisely controlling the ADC's voltage reference?  Any fluctuation can cause large errors.  What precautions have been made with regards to filtering out the ADC's random noise?  For example, many ADCs only have a fraction of their resolution if averaging is not used.