Cruising but engine runs exclusively

I noticed this (abnormality?) today when I was driving on the freeway. I would be cruising and easing off of the gas pedal, going between 55-60mph. The instantaneous mileage on MFD would say ~67-80 mpg. Great. But the Energy Diagram screen showed that the gas engine was running exclusively to provide power to the wheels (dark orange arrows) - without the electric motor assisting (yellow arrows) for a second or two, like it usually does. Why does it seem like my car is using all gas when I'm eased off and the MFD says I'm getting such high mpg. I can attest that something is *different*, because even worse is that my pips are falling fast, so it seems that I am in fact, getting worse mileage.

I replaced my front right tire last weekend (although with the same tire), so maybe the new treads have something to do with it. I don't know. Can anybody help me? I'd rather have this be a new tire thing rather than "there's something wrong with my Prius, it doesn't get the mileage it's supposed to." Like the stories I hear about people getting only 42 mpg.

I hypermile and used to get 54 mpg but if this keeps happening, I know my mileage will slip. What is happening? Thanks!