My investments knowledge is limited, but I think ARKG is an ETF, but not an index funds.

It is actively managed, meaning that specific stocks are selected to be part of the fund. This is the opposite of an index fund, where the goal is to buy the whole market.

That's why a fund like ARKG can sometime outperform the market, or, like recently, under perform. An index fund would match the whole market performance, no more no less.