Slow downloade speed despite good interent speed tests (explain like im kid lol)

[VNTERMU7E]

Hi,
first of all Im totally unaware how anything internet/PC related works so even simpliest explanation is good for me. My speed tests says my downloade speed is over 400mb/s while steam peaked belowe 100mb (now, the top is 50mb/s).
Just want to find out what can I do to mantain some better numbers in this matter, since it drops and holds on something between 20-30mb/s. I obv know that its almost impossible to have that 400mb/s downloade, but steam manages to get close to 100mb and then just suddenly drops to low speed.

[Pscht]

A) There's a difference between Mb (megaBIT) and MB (megaBYTE), which is it?
B) The speed test servers are not in the same location (and thus reached by the same connections) as the Steam download servers

[Elucidator]

50mb/s = 400mbits/s
bytes = bits / 8