What is an ISP actually selling you?
Access to their network, and through it a share of a shared resource — which is why "up to" appears in every advertisement and why your speed varies by time of day.
What the connection is. Your line runs to a local aggregation point — a street cabinet, an exchange, or a cable node. From there traffic joins a backhaul link shared with everyone else served by that point, then travels to the ISP's core network, then out to the wider internet.
Contention. The shared segments are deliberately provisioned for less than the sum of everyone's maximum speed, because not everyone uses their full line at once. The contention ratio describes how many subscribers share a given capacity. This is not a defect — building for simultaneous peak use by everyone would be enormously expensive and almost entirely idle.
It is why evening speeds fall: the peak period is when the assumption is tested hardest.
The three things you are actually paying for:
Access speed — the maximum your line can carry, determined by the technology and, on copper, by distance.
Capacity share — how well provisioned the ISP's backhaul and core are. This is the invisible difference between providers on identical lines, and it is why two ISPs reselling the same infrastructure perform differently.
Transit and peering — the ISP's connections to the rest of the internet, which determine the path your traffic takes and how congested it is.
What the numbers mean:
"Average speed" in UK advertising must be a median measured at peak time for at least 50% of customers, which is a stricter standard than the older "up to" claims.
Sync speed is what your router negotiates with the line; throughput is what you actually achieve, always lower.
Symmetric vs asymmetric — most consumer connections give far less upload than download, on the assumption that consumption exceeds production.
Traffic management policies must be published, and describe any prioritisation or throttling applied.
A business line costs more for guaranteed bandwidth, a service level agreement with compensation, and faster fault repair — not for more speed.