Imagine a row of baths fed by one main pipe with branches. If no one else is bathing it fills quickly, but if many are wanting a bath at the same time there is only the same amount of water for all to share. Some ISP's seem to be able to supply at higher pressure!. My 8 Mbps varies from about 2 to about 6 Mbps. I use Speedtest.net
to monotor speeds.