Someone can call me out for being "dof" about this, but I don't think "mbps" is a thing.
I think the correlation is between "Mbps" (Megabits per second) and "MB/s" (Megabytes per second).
So, in your 1st eg. the 17 - 20 is Mbps, and 1.5 - 2.0 is MB/s.
The 2nd eg. I don't think translates at all, other than the fact that the speedtest & where you were d/loading from had two vastly different speeds because likely the server you were d/loading from was running slower for some reason/s - situated o/seas other than Western Europe / UK which I find is good, usually for d/loads, congestion, faulty at the time etc.
Hopefully someone else who knows more can add to, or correct this.