I have a monthly transfer for 2000GB. I rarely break 80GB. This is for $6 a month.
Yeah, overselling, and is done by 90% of hosting companies. The reality is 2000GB a month costs a lot more to maintain in a DC but people don't meet it. They'll have 4,000 other accounts like you that only use 80GB or less a month, thus overselling is born.
OCUK gets thousands of views a minute, if you happened to be a webmaster with a shared hosting account on a small site and you stuck up a 500kb picture you thought was hilarious and it got posted on here you'd soon find your bandwidth stacking up. And then the server also has to deal with the image requests, which means you might go over your alotted CPU/Memory allowance (small print), which means you might go slow or crash.
Hotlinking on a site that's not designed for it can have big implications if it's being hotlinked somewhere with substantial traffic. It's a bit like sticking your foot out on a motorway.