A lot of companies likely use the auto line speed management, where they poll the averages and increase or drop the speed based on 2-3 days polling loops, maybe thats related to the issue.
To be fair its going to be very hard to get accurate speed info when sharing connections, there so many sneaky backdrop apps that use bandwidth... little stuff like automatic syncing for services, let alone malicious ones. Being on wireless rather than ethernet wires will also give bigger margins on the connectivity as the signal can be disrupted by all sorts of household things, mainly cordless phones and the like, even having neighbours on broaband next door with wireless devices can disrupt speeds, which requires changing router channels to find one with the least amount of noise... although thats a trial in itself as theres no way to gauge it's clarity.
As for gaming online, well as a very avid gamer... up/down speeds vary quite a lot depending on the games... some are optimised for small useage like 10-25k up and 20-50kb down but some can be 100-300kb down and 40-50 kb up, factor in things like having any chat programs like msn or skype, even networked folders syncing etc... suddenly theres lots of avenues for slowness on shared connections, add to that some routers perform better than others with regard to "overheads" when everyones doing stuff online, hell, even heat can mess with a router so make sure its vents are clean etc.