Pretty much all cars act that way. There are a number of factors, the two most important are your current average MPG reading, and the average reading of the fuel in your tank (fuel sloshes around a lot and if you're driving in hilly and/or winding roads the amount of fuel registering in the tank might be lower than actual).

 

and the opposite happens when you fill up your tank. When you first start out range might say 350 and will drop based on MPG (ie. 275), but if you jump on the highway and you MPG climbs, range will jump up as well. Range isn't an absolute, but have found it to be directionally correct.

 

 

I'm guessing that your wife's MPG is fairly consistent and that is why you seeing a linear decline in range as she drives. I'm confident the scenario I laid out above is happening in the Lexus as well.