I figured I'd interject my two cents in case it might give curious people some knowledge to figure out why this happens.
In my case, I'd had the CEL come on about 10 days prior, and had checked those as the 0420 and 1420 codes. I'd intended to go to the dealer and have the 07-036 TSB software update performed, but never got around to it.
So as I'm driving to work yesterday I get the eratic behavior, which at first I thought was awesome mileage, until I started climbing a hill at 35 and it was still at 150 mpg. Upon stopping at a light it went to zero, and wouldn't budge from zero until I got up to speed, at which time it went to 150. The IMA still seemed to be working correctly, so I didn't worry too much. About ten minutes of this went on until I forgot to turn off climate control coming up to a stop light so the car didn't auto stop. So I wound up shutting off the ignition and restarting the engine once the light turned green. Upon restarting, the bar graph started working correctly again, and the CEL was gone.
It might be coincidental. It might not be. Is it possible that every once in a while the software recalibrates or reboots, and while it is doing so the first start-up has wonky mpg numbers? I can't say I'm all that vested in figuring out the why of the phenomenon since it doesn't seem to be detrimental to the car, but figured I'd throw my piece of data into the mix if others care to mull over it.