ICE stands for Internal Combustion Engine.
As for "inaccurate"... the built in display isn't dead on but the drop in mileage because of your 5mi trips is to be expected. A cold engine isn't nearly as efficient and it isn't surprising that your short trips get lower mileage. Keep in mind that combining mpg numbers can't be done by a straight average -- the fuel usage is in the denominator of your ratios (miles / gallon) and so the lower numbers affect the average much more than you would expect.
As an example, you might expect that driving one mile at 100mpg followed by one mile at 50mpg would yield 2mi at 75mpg. In actuality,
1mi/100mpg + 1mi/50mpg = 0.03g used
2mi/0.03g = 66.7mpg
The reason? You used twice as much fuel driving the distance at 50mpg as you did driving at 100mpg.