Okay, first of all the naming: a MB (Megabyte) is different from an Mb (Megabit) and an mB (millibyte) doesn't exist- it would be 1/100th of a byte, and we can't have fractional digits in a number. There are 8 bits in a byte, so 19Mb/sec (your reported speed) is about 2.4MB/sec.
As to why you get the slow speeds: When people say the Internet is the "information superhighway", that's actually a fairly apt description of how it works. You have the Internet backbone (the highways) that connect regions and networks together. That stuff is all super-high speed. Then there's the smaller roads- the points where information goes from the backbone to your home. The 19Mb/sec measured is the speed limit going from your house to the main hub. But there's a lot of road to travel between your house and the servers all over the globe, and if even one of those lines is congested (or has a less than 19Mb/sec available), you'll be slowed down.
There are ways to get almost the full bandwidth filled though. Really large companies like Adobe, Microsoft, Apple, and Google have way more bandwidth than you do. I've gotten great download speeds while downloading large files from them. The other option is torrents. I don't mean that you should start stealing stuff, but the concept works well. You have multiple servers hosting the same file, so you're downloading from multiple places at once. Even if all 4 or 5 lines get slowed a little bit, you'll still be able to use most of the 19Mb/s. I know Microsoft uses something similar to this for downloading large files. Many open source programs will post their own torrent links because this lowers their bandwidth cost and speeds up the downloads for the users.