Why are the ERP amplitudes recorded and analyzed from different software so different in voltage? Here are the two ERPs from the same study design, though different subjects. The largest amplitude in the 1st one goes up to 25 uv, while only goes up to 6 in the 2nd figure. 

Differences: The 1st one were recorded with 60-channel Easycap, analyzed in Brain Vision, nose referenced; while the 2nd one were recorded with EGI 128-channel Geodesic Net, analyzed in Net Station, averaging referenced. Are these sufficient to explain the big difference in the voltages? Thank you!
The short answer is yes, probably. 

Referencing is the process of subtracting a referent signal from all of your channels. While recording, the EEG data are typically online referenced to a vertex electrode, earlobes, back of the neck etc. During post collection processing EEG are most often rereferenced to the average reference, or the linked mastoids. The reference  that is selected changes the shape of the signal simply because the reference channel (or average signal if using more than one) are subtracted from every channel. This means that comparing results across data that have different references is not direct. Since referencing is linear it can typically be undone and the reference channel can be changed. I never use a nasion reference so I can't say for certain why the amplitude is so large but I would guess if you looked at the signal at that channel you would see how it is adding (if channel of interest is positive and referant is negative then amplitude goes up) to your electrode of interest or subtracting from it (if both are positive). For a P3 response 6 micro volts is reasonable. If you want a more detailed explanation of referencing I recommend Nunez's book, or another book on EEG data analysis and theory.