P3 ERPs are not consistent between different channels

Dear Emotiv

I was reading a research paper on https://www.emotiv.com/independent-studies/p300-and-emotiv-epoc/

One of the figure in the paper, as shown below

channel 7 and 8 (o1 and o2) has negative p3 potential. but some other electrodes such as channel 12, 13, 14. (F4, F8, AF4) has positive p3 potential.

Could you explain why it is not consistent? is it because P3/P4 reference electrodes in between of (p7 o1 o2 p8) and (T7 FC5 F3 F7 AF3 T8 FC6 F4 F8 AF4).

Thanks
Seany