If you take apart a Burnt carto you will more then likely see what the reason is. A carto has a cloth like piece that is wrapped around the heating coil, then polyfil is wrapped around that. If it is heating, but giving you a burnt off taste more then likely its the cloth piece that has burnt up and you are getting the flavor from that. Also if you take apart a burnt out carto you can see the junk that can build up around the coil. It is from anything that doesn't completely vape off, such as flavors and such. Its just the way that cartos are, as they are not ment to last forever. The reason LR cartos are more prone to this is because the nature of the LR, they burn hotter, and its easier to burn the cloth without topping off every so often. They also vape more juice then SR so there is more "junk" left quicker. Hopefully that answers what you wanted to know. The best way to see what im talking about is to take apart a burnt carto and see it for yourself.
And to answer your question as for what the cloth like piece do and the polyfil, I think (could be wrong) is that the cloth around the coil holds the juice "tighter" around the coil, and the polyfil is a better holding/wicking material.