Shows TRX with BTT, subtracted or added to. Subtracting makes it in line with BitMEX futures. The option to add is considered for 'true value'. Airdrops, like new coin forks, can split value: a good example being BCHABC and BCHSV.
I'm guessing it has something to do with (spot price * estimated airdrop rate) trading too far from futures. Maybe even too close. Or because futures are trading above spot near a monthly airdrop snapshot. Much complicate. Such guess. Wow.
Binance is chosen as pricing source for its liquidity and earliest open date.