BitMex XBTUSD price

[CMCET]

I'm not really new to crypto or trading but have never actually traded with leverage.
Have recently opened a demo account on BitMex, but i can't figure out how the XBTUSD price is calculated.
it's stated that XBTUSD is a XBT/USD perpetual contract priced on the .BXBT Index and .BXBT Index is 50%/50% combination of BTCUSD price on GDAX and Bitstamp.
The thing is, if Bitcoin price (so also .BXBT Index) goes up with, say 3% , XBTUSD goes only up with about 1.5%. How does that work?

Another thing is that you can't find the XBTUSD price on Tradingview. The XBTUSD price for BitMex on TradingView is the perpetual inverse swap contract price wich is closer to the .BXBT Index price, but differs from the price that's used when opening and closing positions.
So is the only way to see the XBTUSD BitMex price from their own chart on the website?

[Crashing]

Bitmex price is calculated taking in account those two prices but it moves like any other price. If the price on the two exchanges moves 3%, its price will move 3%.
It might be worthy to check: http://japnsurplusman.blogspot.com/2018/08/bitmex-next-generation-of-bitcoin.html