[trendia]

Serious question: What would Comcast do that would cause that?

Would they give you a higher proportion of bandwidth when you first subscribe for internet service (and checking speedtest.net), eventually reducing it to give way to other new subscribers? Or do they just not perform sufficient maintenance to keep the high speed?

[guelo]

I could see this happening if there is an oversubscribed uplink that gets worse as more subscribers are added over time.

[rhizome]

Both. Overselling has been common for 20 years and leads to congestion when customers exceed the percentage of their likely bandwidth usage as estimated by the company.