Saturday 3rd - Just when I thought I understood it

Just when I thought I had it figured, I get new and even stranger behaviour from my Volt. Back when it worked properly I used to get somewhere between 10.1 and 10.8kWh power used from a full charge. This varies a bit because it's only an estimate, but I've never heard of anyone getting over 10.8kWh without some top-up charging during the day. Then on Saturday I apparently got 11.4kWh, except I don't believe it was anything close to that.
On Friday, BTW, the Volt stayed in the garage, so it isn't that I've missed a day, just that there's nothing to report.

Saturday started with a 49 mile range estimate, 11141 miles on the clock. We drove to Oxford (10 miles, 37 miles estimate remaining).

 The plan in Oxford had been to recharge, but a single car had managed to ICE both charging spaces, so when we left the numbers hadn't changed:

We came home along the A34 so 13 miles and several of them at 70mph which reduced the estimated range remaining down to 20 miles. Normally at this point I would have plugged in to recharge, but as I hadn't managed to charge earlier, and interesting things happen when the charge gets low, I didn't plug the car in.

Despite not being plugged in the range has jumped up quite a bit when we leave an hour and a half later to go shopping:

 4 miles to the shops, 4 miles off the estimate:

Range pretty much unchanged while at the shops:

 Home again. 4 miles driven, 8 miles off the estimate. My wife was driving and commented that we nearly got home with 17 miles remaining, but 2 miles disappeared in the last couple of hundred metres.

I left the car as it was for an hour thinking that might trigger the 10 mile drop I've seen several times now, but in fact the range stayed steady at 15 miles. Also at this point we've apparently used about 7.9kWh for 30.7 miles (average 0.26kWh per mile). Time for another shopping trip.

I pulled over less than a mile from home because seriously strange things were happening: the range estimate was steadily counting down every few yards, and the kWh estimate was shooting up, at this point nearly 1kWh in under a mile. During that mile my speed never exceeded 30mph (in fact it probably never got much over 20), and it was dead flat as well.

The counters settled down a bit after, so here I'm 4 miles from home with an estimated 6 miles remaining, but apparently those 4 miles took over 2kWh. In fact it was worse than that because most of the 2kWh went in the first couple of miles but I didn't have anywhere to pull over sooner for a photo.

Shopping done, can I get home (about 5 miles), on 5 mile estimated range?

Stopped at traffic lights, nearly home, I might just do it before the petrol engine takes over:

I got home with no green bars showing, but still on battery. The kWh used estimate is an unheard of 11.4kWh, but given that I got under 40 miles from the charge I think it is way, way off the mark.

39.5 miles covered, but I don't believe the 11.4kWh, let's see if I can guess what I think it should have been.
The morning estimated range was 49 miles, if the battery gave me 10.8kWh that would be 0.22kWh per mile. The 8.8kWh used I've seen several times at 0.22kWh per mile would have given me 40 miles, so those numbers all seem to fit close enough. I think that despite the crazy 11.4kWh figure on the display I probably got under 9kWh from the battery again.