Why is my Analog Input channel adding voltage to my signal?
Wow, you guys are gonna get a KICK out of this!

The analog input channel was not adding millivolts to my Analog Input signal. I stupidly was not supplying my Arduino Mega with 9V!! I had it running off of the USB port, which was affecting my 5V reference (thank you @majenko for pointing that out!). 

I measured the 5V pin and it was fluctuating between 4.41-4.21 volts. My colleague made a comment and I reviewed the Arduino Mega datasheet. It must be supplied with 7-12 volts in order for the 5V to output correctly. 

Once I hooked my Power Vin + GND to my power supply, set that at 9V/.08amps my analog input readings were accurate. 

Thank you everyone for your help!