How come the P/L is different than the reference price of the ETF itself?

I'm confused about this. I'm sorry i'm very new to this. I know ETFs are meant for a long term investment. Does that mean the demand for this ETF is just low when the reference price is dropping, but the performance is not as bad when P/L didn't drop more than 1%? Meaning if the demand for it is low, I would have to sell it below the “Performance Price”?