My home ISP gives me a certain number of megabit up and another down. My contract essentially says 60up/20down unlimited, which means that I can download as much as I want, subject to the limitation not to download more than 60 megabit a second.
We could use that as a template for 3g sales as well - granted we may have to specify the speed as 3g or 4g rather than a certain number of megabits - but ISPs hate that they are selling a commodity (which they shouldn't. Butter, toothpaste and oil are commodities but Procter and Gamble, Unilever and Saudi Arabia are all rich).
I would say "60up/20down unlimited" means your ISP guarantees this bandwidth as part of the contract, rather than upper bound the bandwidth by this number. In the off peak hours you may be allowed to download at a rate higher than this number, but even in the peak hours you should be able to fully utilize your guaranteed bandwidth.
Even a "guaranteed bandwidth" becomes impossible if you're transferring from a server that can't provide those speeds, or over some portion of the Internet that can't due to link congestion/etc., and your ISP can't do anything about that. In that sense, all bandwidths are "up to".
The significantly more expensive "business level" plans usually do have some sort of guarantee via SLA, but once again that only applies to the part of the network between you and the ISP.
We could use that as a template for 3g sales as well - granted we may have to specify the speed as 3g or 4g rather than a certain number of megabits - but ISPs hate that they are selling a commodity (which they shouldn't. Butter, toothpaste and oil are commodities but Procter and Gamble, Unilever and Saudi Arabia are all rich).