[SkiFire13]

This doesn't prove anything, it just means that when you're doing a speedtest there aren't other users that are also saturating the connection.

Note that this doesn't means that there are exactly 0 users downloading in those moments. Usually there are multiple gigabits dedicated to a group of users, so that multiple users can navigate at 1Gbps without slowing down others, but not all the group at once. How much bandwidth is allocated to how many users can vary though, and some providers might allocate less total bandwidth to more users.

In practice this works out fine most of the time and most users won't notice slowdowns like you do, but if everyone started a speedtest at the same time you will notice it.