Guarantee? What guarantee? Both DSL and cable internet service are provided on a "best effort" basis. If you want a SLA, you have to pay through the nose for it. Guaranteeing a SLA means the provider has to provision dedicated circuit capacity, instead of letting you complete for channel space on a first-come, first-serve basis.
With DSL, the uplink and downlink depends on the DSLAM-to-CO channel capacity, because DSL is implemented using ATM and virtual circuits in fiber rings. The differing up/down rates are a design decision, based on how many of the sub-carriers are assigned in each direction. Oversubscription is the carrier's choice.
True cable service is another story. The downlink is managed by the head-end, so the feed onto the cable can run at top rate. Yes, the more users who are on the subnet in your neighborhood, the slower things can go. The uplink, however, is a single channel shared by a number of sources, so the upstream channel acts like AloahNet back in the 60s: a fractional load can saturate the uplink because of contention. (ThickNet and ThinNet suffered from the same congestion problem...which is why most people use twisted-pair star networks, even in our homes.)