The Ethernet throughput read-out can be pretty close to the actual user data throughput. Its read-out is always a little higher as it includes overhead data, such as for routing, handshakes, error checking/correction and so on. You can see this for the speed it shows in the other direction, i.e. the 0.3Mbps upload shown above is mainly overhead data as this test is taking place. So around 0.3Mbps of that 30.5Mbps is likely overhead data also.
If let's say you run this over Wi-Fi or there was a poor Ethernet connection, that figure could be a lot higher than the actual throughput, such as if much of the data over the Ethernet connection to the Router consists of retransmissions. It was one thing that used to bug me with the Ookla test as it would filter out dips that accounted for less than 30% of the test duration, giving inflated test results over wireless connections.
If there's a server on the network (e.g. workplace), the Ethernet throughput will include data that occurs over the local network. For example, Windows will periodically index network shares, Exchange databases and so on. Windows 10 also distributes Windows updates between PCs on the same network. Network devices that broadcast data (e.g. printers) will also add to the Ethernet throughput figure. Then again, running an Internet speed test at workplace can also be challenging as it's difficult to make sure the Internet connection itself is idle at the time of testing.