I wonder if the error in your EV range was caused by the system learning too much from the grade and thus underestimating what you had left.
I'm beginning to think that on a long trip it may be best to use the HEV mode until in EV range of a charge. The little 1.6 needs electric help on a grade.
My last two charges have been 9.0 and 9.4 KWH's respectively. With 10% or so left, the battery has to be 10 KVA plus whatever overhead it allows at full charge to preserve battery life. If I drive around town, I always get 26 or more miles in pure EV. REALLY refreshing to see a company fairly representing performance figures.
I really don't know what that was about. Maybe the ICE was running in generating mode after I switched back to EV mode at the top and I just didn't hear it. I just noticed that as I continued to drive at 70 MPH for many miles, the EV range didn't budge. I parked that night with an EV range around 4 miles. When I left the next day, it was around 6 miles if I recall correctly.
When I come back down that same route, I've been disappointed at how little EV range I pick up: only a couple of miles at best, while the car is in HEV mode. I did a different mountain drive at Christmas time over "the Grapevine" headed north from LA. The EV range dropped near the summit, but I kept it in HEV mode and unlike my Cajon pass trip which levels out at the top, the trip over the Grapevine is immediately followed by a steep descent. On that trip, I recouped most of my lost EV miles on the way back down, maybe even to the point of having a couple more than what I started with. Of course, I was in HEV mode the whole time so it's not like it was "free".
Regardless of the explanation for what reads on the EV range, I no longer use up my EV miles at the outset of a long trip. I use up a few so that the battery will have room to accept charge from regenerative braking, and then I switch to HEV mode. On the way home, when I get close to home, I switch to EV mode to use up any miles I have left in the EV range. And if I get stuck in stop-and-go traffic along the way, I switch to EV mode for that too. Having that surplus capacity along the way helps when you want a little more power, or have to struggle with stop and go traffic. And it's just as easy to use it up at the end of the trip as it is to use it up at the beginning - well, almost as easy: you do have to remember to manually flip the switch.