The calculator in the 4XE has a crazy imagination and a bit of a sense of humor. It also has no way to measure the energy used when in electric mode. It calculates the miles but not the electrons.
it starts your calculations based on where your overall average is. So if your overall average is 30MPG, it sort of starts there. It figures 99mpg while driving in electric. Somehow, it weighs that 99mpg against your overall average. The more electric miles you cover, the higher that overall weighted average is.
I was messing around one night as I drove in electric mode. I kept resetting both trip odometers and the overall average MPG over and over. I eventually got my overall average to read 99mpg and that’s where it caps out.