somewhere, between you and the server you are connected to, the bandwidth is shared.
But the difference here is that on a fibre connection the shared portion goes over higher speed trunks which gives you most of that 1Gbps bandwidth. A wireless connection has a limited number of slices in the same band that it can share.
It's the same issue with too many people on a single WiFi connection.
I thought that guy AL Jippity was just doing a ton of work. Like a 10,000x dev.