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.