I was wondering, why does people count dps instead of total potency?

[KaiSunstrider]

If you do 738000 potency in 3 min with your samurai with 4100dps because you're ilvl 300, and another one does that same potency of 738000 with his samurai with 4400dps because he is ilvl314

I just dont understand i guess. How can you do 738,000 potency in 3 mins and with 4400 dps when 738,000/180sec = 4100 PPS? It is impossible to do the "Same potency" in the "same amount of time' and have diff PPS numbers. PPS = Total Potency done/Total amount of Time. If two people do the same potency in the same amount of time, it is mathmatically impossible for them to have different PPS/DPS