My vote is on forced regen. I have had this happen and it almost always correlates to me not being able to get the last bar of regen charge off of the meter without kicking the ICE in more and dropping the mpg even further. This is really obvious with low SOC (4-5 bar range) as the car demands 3 or 4 bars of forced regen, but I also think the computer monitors you total inflow and outflow of current to the battery and prematurely stops the system from going into a negative sum game at higher SOC situations (ie forces regen to keep the SOC at the steady state high level). Most of the time this happens to me is when I am traveling without many breaking opportunities or if I am not using hills to regen, but to coast more.

The only other time I see a lower than normal mpg than I would expect is if I did not let off the gas pedal long enough to drop the rpm's down all the way, thereby getting the CVT into effectively an overdrive gear ratio. I have gotten into situations where I was running at a higher rpm than I really needed to for that situation. Is it possible that you are not fully letting off the gas and allowing regen to kick in before applying the gas again to get into the lowest mpg range? Watching the RPM gauge seems to me to be the best way to verify what the mpg should be reading if you are not bogging the engine down.