Generally, if you pay for ISP for 30mbps you won't get that. And even if you get a high result from something like www.speedtest.net
there are still many other things that can effect your downloads. As other people have said, your distance from the main lines and how many people you share that with messes around with things like line noise, throughput/trueput and a whole array of possible interference. On top of this, you might be able to download a file at 50mbps, but if the guys your tying to download off can only upload to you at 1.5mbps then you won't use the full potential of your internet speeds, especially if the people you download things off also have another 1000 people trying to download from their servers. AND something ISP's love to do is throttle your speed, if you are the kind of person that likes to download things and you went for that "Unlimited downloads" from your ISP, you might want to check that fine print because most will kill your speed down to barely anything as punishment for using your unlimited downloads
a year or so ago, i downloaded around 200gb in one week using BT's unlimited fiber-optic broadband and i got an email from them saying i exceeded my fair usage and if this happens again then my 60mbps would be throttled to 1mbps!
i also saw someone said about the megabits/bytes, i think it would be very cheeky and misleading for them to advertise in this way
I've only done one semester of networking so far so what i say might not be 100% correct, but it should be about there.