...why is the battery called a KR808D-1?
It's just a model number, specifically the model number of the original Kanger device that all the other compatible devices are based on.
Think of it as IBM-compatible (those born after the '80s might not get that reference though). Or more technically, the 8086 PC standard. The coaxial audio/video jack standard is called RCA, for the company that first used those. What does IBM stand for? Why 8086? What does RCA stand for? Philips head screws?
Who knows where these companies got those names from?
Well, we could probably look up most of those. But, point is, they're just names. When you make something that catches on, and other companies want to make things that are compatible with it, they let everyone know their products are compatible by referencing the name of your original product. These names are a short way to let us know what to expect from a product.
We don't need to know where the names originally came from. In fact, we're probably better off not knowing. Such knowledge could be disturbing and dangerous.