That makes sense, based on where and when it happens to me. The behavior is exactly like when the engine warms up and it only happens when I've been gliding a lot.

I'm a bit confused on the benefits of "gliding" versus regenerative de-accelleration. When I am driving down a hill, I let off the gas and let the car cruise. If I start to lose speed, I go into regenerative mode. And the mpg bar says I'm getting 99.9 mpg. But even if I am gliding in the pure sense discussed here, I still get 99.9 mpg. Is there some advantage to gliding without regeneration in terms of mpg, and if so, what is the benefit?

Gliding, as I understand it, uses absolutely no power or gas... you're coasting. Therefore, you get quite a bit more than 99.9 mpg while gliding. The display shows 99.9 mpg because that's the highest it will go. You'll find out what you really got when you get to the pump for a refill.

OK, I see. So essentially I will not be seeing any change in my average mpg on the screen, but I will see a better result on total miles driven on the tank? Will there be a discrepancy between total miles on the tank and the average mpg? I am guessing so, but I may be missing something.
I've found that I am better able to glide when its pretty hot outside and I keep my foot on the accelerator just ever so slightly.
I will test this out and see how far I can stretch this tank.

The display only goes up to 99.9. Your actual instantaneous MPG goes much higher. Witness the guys who got 112MPG on a single tank of gas, last August, using the P&G technique.

When you're gliding, the only energy the car consumes is to power the electrical system. This means the computer, headlights, and whatever else. When you're coasting, you're doing much the same, except the regenerative brakes are engaged. You lose momentum, as the brakes regenerate energy.

Use the two techniques where appropriate, to maximize your MPG. If you see a red light ahead, coast into it. You're going to have to stop anyway, so you'd may as well regenerate energy with the brakes along the way. If you're in stop and go traffic, you can glide a lot, as you approach slow-moving traffic. If you're a hard-core hypermiler and want to maximize your MPG, you can pulse up to 40MPH, then glide back to 25 or 30 MPH.

Think of it this way; Gliding is free MPG. You get nearly infininite MPG. Regenerative braking is not free MPG. You're inefficiently converting kinetic energy to potential energy. It's better than pressing down on the accelerator but it's not free. Whenever possible, you want to glide, rather than coast.