Yeah, it took some digging for me to see the difference. Here’s a scenario:
1. Use USD to buy 1 BTC at $5,000.
2. BTC goes to $10,000. Sell 1 BTC to USD.
You have a 2x USD gain.
Now, BTC price moves up to $12,000 and you buy back in.
You can only buy 0.83 BTC.
So while your USD value has gone up 2x, your BTC has dropped from 1 to 0.83.
It’s all interchangeable money, but there are people who want to trade in a way that specifically targets building the BTC quantity.