Or are my expectations for "highway miles" too optimistic?

My DEF warning light came on about 1/3 the way through a 1000+ mile round trip, which was a bit unnerving until I read that I could refill it myself, if necessary. What I did find surprising is that my fuel mileage was atrocious on mostly highway miles, in fact worse than on the same trip in early January during bitterly cold and sometimes blizzard conditions.

I've been tracking my mileage on fuelly.com for most of the time I've had the vehicle, so looking at how it has performed over time is easy enough. Most of the time I'm commuting with 40-50% city driving and averaging high 22 to mid 23 mpg. That seems decent enough as it is as good or better than I was getting in my e92 335i.

On the recent trip we were averaging 80 MPH on I76 and I80 in Colorado and Nebraska, but the X5 couldn't seem to manage more than 22 MPG, which was surprising as I was expecting 25 to 26 MPG. In January when the temps were below freezing, with more people and luggage, and with a strong headwind the X5 was getting 24 to 25 MPG. The only obvious "negative" on this latest trip was that the DEF warning light was on. Maybe the X5 changes the tuning a bit to conserve DEF, negatively impacting MPG?

Or am I simply expecting too much? Maybe 80 MPH is just too fast for the gearing?

I'm not trying to "flame" the X5, so don't get me wrong, but the poor MPG caught me by surprise!