Late to the party as usual, but with a topic I'm familiar with.
Up until last year (2015 was last year, right?) I worked for Xplornet. Not in sales, or phone support. In the back end, operations/engineering side so this is the straight poop. Warning: wall of text.
The phrase you're looking for is oversubscription rate.
What happens, in simple terms, is that there is say capacity for 100 Mb/sec aggregate throughput to you. Let's ignore the upstream since that doesn't come into play very often as a bottleneck unless you're sending photos to the cloud. So you pay for a 20 Mb/sec service, you're throttled to that rate. If there is you and four other customers, you can ALWAYS get that 20Mb/sec rate, since 20 x 5 = 100. So the five of your could stream 20 Mb/sec all day every day without interfering with each other. That's a 1:1 subscription rate, and you do NOT want to pay what it would cost.
ISP's count on not everyone pulling data at the same time. So say they put 10 customers at 20Mb/sec service. If all 10 tried to pull their full speeds simultaneously, nobody would get it. Mathematically you would each get 10 Mb/sec on a 20 Mb/sec package. That's a 2:1 oversubscription rate. BUT! It is unlikely that all 10 would be online exactly the same time. You pull a web page up at 20 Mb/sec, and while you are reading it, your 9 other neighbors stagger their browsing, so at no point is the access point being asked to deliver more than the aggregate it's capable of and everyone is pretty well happy.
Enter one of the shadier business practices. Say for example, the technology vendor says "based on our experience, a maximum overscription rate is 10:1 for a reasonable experience." The ISP says "well we can cram it to 25:1, charge everyone for the max and coin it! Besides, we advertise speeds as UP TO so we aren't lying." So when you're home in the middle of the day, all your neighbors are out, everything works since their connections are mostly idle. Kids come home from school, fire up the Xboxes, now everyone is competing for the same limited resources and the quality goes to shit. Also, say for example you're on a lightly loaded access point that is directed towards you. Your 90 year old neighbors don't generally USE the internet so you have free reign. And that's why when it works, it works well. When it doesn't it's shit.