Some ISPs throttle bandwidth based upon the origins and destinations of packets. When a person is using a lot of bandwidth for nefarious purposes, the ISPs will either (a) handle the matter internally, or (b) notify the proper authorities that certain transactions have been taking place.
ISPs have all the more reason to invoke their rights and powers to neuter a person who is abusing the network. The moment the OP said that s/he called their ISP to tell them that their torrenting speeds weren't desirable and also notified that part of the bandwidth's use was for acquiring movies, games, and software was the moment that the ISP raised a red flag on the user.
So, then, how do ISPs figure these things out?
(1) The user visited "Thine Pyrite Tray" (a purely fictitious website which is solely for the purpose of illustration). There, they request to download a torrent file. Packets, by nature and throughout the process, are encapsulated or stripped and are properly routed between hosts and destinations. Result? The ISP can furnish indelible evidence that there was traffic.
(2) You can torrent only so much. Let's say you're a big Linux fan like me and you choose to torrent because you find the downloading process to be expedited more quickly. How many distributions can you download before the ISP is alerted?
(3) Let's devise a theoretical account of traffic. To begin, we'll define the test period to consist of a one month long trial; we'll defined one month to consist of 28 consecutive normal (not business) days, involving someone we'll call "Bob" (i.e., our fictive test subject). Every day, Bob downloads a movie which weighs in at, say, 3.5 GB (we'll use this number because the individual is assumed to be finicky about quality). Let's also pretend that in a given week, Bob downloads 2 GB of software (I arrived to this figure by calculating the mean-average of actual package sizes I happened to encounter in my investigative survey. Lastly, Bob is crazy about video-games, so we'll assume that he downloads, at least, one game per week. We'll assume that each game is a healthy 10 GB.
So, how much ideal downstream bandwidth is Bob using? That's pretty simple math: (3.5 GB of movie acquisition traffic per day * 28 days/month) + (2 GB of general software acquisition traffic per week * 4 weeks/month) + (10 GB of video-games acquisition traffic per week * 4 weeks/month). This results in 146 GB of downstream traffic in torrenting alone per month.
Now, while it's obviously given that Netflix is not torrenting, it's important to note that adding Netflix to the mix can easily push this up another 100 GB per month. This means Bob's downstream gobbles up 250 GB per month.
Does this raise a major flag? YES. And, if you call them and tell them you're torrenting then they will monitor your connection with more bravado; likewise, they'll adjust your speeds.