This is what I initially thought too but I also factored in my kitted up weight (closer to 80kg plus the weight of the battery and motor.You
You sound hugely oversprung for 74 kg. I was going to get a 550lb for my weight at about 97kg according to the spring chart.
It does feel stiffer than the stock spring but for some reason the performance feels way better. For me this setup has done wonders for my riding which I would say is on the more extreme side. I was going to try a 500lb spring but the bike is just so dialled in atm.