Android status bar symbols
It is mostly because status bar icons vary from OEM to OEM. Each one has a slightly modified icon set to go with their overall skin/UI. And then icons will depend a lot on the features incorporated within a device. So, developing a full list of icon sets seems like a tough task. Because you'll eventually miss at least something used on a phone.
Other than that, 3rd party apps also show their icons up the status bar. And it's impossible to keep a track of each and every app.