Basically on mobile broadband, Ookla shows peak, tbb shows what looks real from visual analysis of that other monitor.
Ookla doesn’t show the highest speed achieved within the speed test overall, however will manage the connection to try and maximise throughput. It takes the maximum speed achieved right at the end of the test to present back as the achieved figure. This could be lower than the maximum seen within the test itself.
For anyone interested this is how Ookla speed test operates for download:
The client establishes multiple connections with the server over port: 8080. The client requests the server to send an initial chunk of data.
The client calculates the real-time speed of the transfers, then adjusts the chunk size and buffer size based on this calculation to maximize usage of the network connection.
As the chunks are received by the client, the client will request more chunks throughout the duration of the test.
During the first half of the test, the client will establish extra connections to the server if it determines additional threads are required to more accurately measure the download speed.
The test ends once the configured amount of time has been reached.