Brightness Difference between 3.3v and 5v?

While working on two different projects, I noticed a brightness difference of the RGB LED built into an encoder.  My guess is that this brightness difference is because one project is powering the board with 3.3v and the other with 5v.  I would have assumed that 5v was being stepped down to 3.3v internally, but maybe I'm wrong?  Does 5v result in a brighter RGB LED in the encoder?