Internal Aref, 1.1V, is good for lower level signals.
1.1/1023 = 1.07mV per step vs 4.88mV/step if Vcc = 5.0V.
Vcc may vary, especially if from USB, which is spec'ed at 5V +/-10%, so 4.75 to 5.25. Plus there may be varying loss thru the PTC fuse, which has some resistance, thus a varying voltage drop across it depending on current load for the rest of the board.