Simplifications allow a permissible level of error to enter into the model in the hopes of making the it simple enough to run quickly, the concept understandable, and the results readable.
Many thanks to Mike Cowan (author of the Cowan LRA Forecasting Model) for pointing out an error in a prior version of this post, and for providing the numbers behind the chart.
In fact, if you try to, say, call up a Web page while on a voice call with the Verizon model, you get an error message warning the two things can't be done simultaneously.