I’ve noticed that Steam for Windows displays download speed in mbit/s while the Linux client displays it in mbyte/s, although both display the unit as mb/s. This is a setting that can be toggled. This doesn’t account for the entire difference (1887mbit > 109mbyte) but is one contributing factor.
Ah, that’s annoying. Have you reported the issue to Valve? Seems like it’s their issue to fix. I personally haven’t encountered this issue, so I’m unable to help further, but it seems that it’s an issue with Steam’s Linux client since the rest of the system is unaffected, as I understand it.