Maybe just more people on line. You've got physical constraints on the connection (crappy wiring, cheap filters for example) some of which are under your control, you've got capacity constraints (at some point not far from your house you're sharing bandwidth with an increasing number of people), you've got traffic shaping constraints (the provider artificially messing with different types of packets to delay them), and then you get hard capacity limits at every piece of equipment between you and your destination.
For an example of artificial load management, I've got 100Mbps fiber to my apartment on a "home" plan. Everyone else has 100Mbps fiber to their apartments on an "apartment" plan. I pay about $10/month more than the others do. Speed tests show we all have about 65Mbps up and down to the exchange. The difference is, I have 65Mbps to the box in the basement, and 65Mbps to the exchange on a 1Gbps connection. Everyone else has 65Mbps to the box, then share 65Mbps to the exchange. That's done by load management software. In reality, at peak times, my connection seems to be about a bazillion times faster than everyone else.
(My other feed is 1 Gbps fiber to the building on a competing carrier, with 67Mbps VDSL to me. It feels much, much faster than my fiber but is asymmetric.)
Since you seem to be a competently inquisitive person, see what happens when you disconnect every other device from the line. The parent's place couldn't Slingbox TV to save its life, until I found the cheapo handset that was killing the performance, even through its filter.