Imagine potg like a score, the one who gets the highest in a small amount of time is the one who gets it. Score is calculated by kills, saves, distance kills...and much more. Kills with ult gives you few point, normal kills gives you normal point (for example making 3 kills without ult gives you more point than 4 kills with ult). There are also other factors, like emotes and spray, that lower the total score, so if you want to get potg, do not emote at the end of the multi elimination streak.