This is due to the underlying mechanics of how ETFs operate.
If the market price of ARKK shares goes above (or below) the actual value of the underlying basket of shares it represents, new shares of ARKK are created (or destroyed) and sold to (bought from) the public. This forces Ark to go out and purchase (sell) more shares of the companies that are in the underlying basket of stocks that represent the new shares of ARKK.
As a holder of ARKK you are legally entitled to the underlying stocks that it represents. Those underlying stocks are held in a trust, of which you, as an ARKK shareholder, own a percent of equivalent to the number of ARKK shares you hold.
ETF shares are forever being created or destroyed. There is no real reason to keep an eye on the outstanding float of an ETF as it’s not a measure of a scarce resource. That’s the point, it enables the ownership of a basket of goods. The goods are scarce but the baskets are not.
So when we look at the ARKK position chart of Tesla what we’re really seeing is two things:
1. The position size of Tesla within the ARKK fund. A sharp increase or decrease is indicative of a trade.
2. The implied demand for ARKK shares. i.e. If there are no trades reported and the chart is drifting up, demand for ARKK shares is increasing. A downward drift means demand for ARKK shares is decreasing.