Because one figure is reported and the other is computed.

The free data source states the floor price in ETH. That is the currency this market trades in, and therefore the figure that actually comes from outside.

The USD column is produced on our side: it multiplies the ETH floor price by the ETH price. It therefore moves even when the floor price itself has not changed, namely whenever the ETH price moves.

Both columns are shown so you can compare without converting, while still seeing which figure is the original one. If the ETH price is missing, the USD column stays empty rather than estimated.