It is the quantity of bits transferred in a second! Using 1000 rather than 1024 makes the quantity number bigger which appeals to marketers, especially hard disk manufacturers.
You are confusing speed tests which measure flow rate, where 1K = 1000bps and 1M = 1000K, (at least reputable ones do), and file download managers that report the number of 8-bit data bytes downloaded per second. When talking data sizes 1K bytes is 1024 bytes as file storage is all based on powers of 2 for efficient handling on spinning disc.
Because you are told file sizes based on 1024 the download managers report in the same way so you can relate the speed to the amount you are waiting for. When you want to know your bit-speed per second then working with base 1000 is more comprehensible. We all know that 16 x 1000 = 16000, but what proportion of the population know 16 x 1024 off the top of their head?
Only geeks like us.
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