Toyota Prius: Does Prius do this?

06 Highlander Hybrid:

Driving around town,
-on flat & level terrain
-with only the driver; no passengers
-with no cargo
-when not in any terrible rush:
....if the throttle is opened a VERY small amount, the vehicle runs on
battery and the gas engine does not start, which is GREAT!

HOWEVER:
If the trottle is opened just slightly more than a small amount, the gas
engine starts, when there is no pressing need to do so.

I drive _very_ leasurly, coast to red lights, accelerate slowly, never
exceed the speed limit, and yet the vehicle still falls short of the
advertised fuel economy.

Is the vehicle designed to needlessly start the gas engine, consuming more
fuel than necessary, or is mine malfunctioning?
How can I determine whether my vehicle is performing as intended by the
manufacturer, in terms of the hybrid system's behavior with regard to gas
vs. battery power usage?

bicycle6228 wrote: 

Advertised fuel economy is the EPA estimate that most people complain
about never achieving. You can if you drive under the same conditions
as the computer model that is used to determine these estimates. On my
Prius, I find that I get the advertised 60 MPG in slow moving commute
traffic on the freeway (10 to 35 MPH). But then... I have HOV access
so I don't end up that situation to often.

(I still find it ironic that us small hybrid owners get special access
to HOV lanes when we get our best gas mileage while stuck in traffic)

To answer your subject line question: Yes, the Prius does the same
thing. As mentioned by a previous poster, a major factor is emissions.
Particularly the catalyst - it has to stay very warm to function
properly. This is one of the reasons, in addition to changes in gas
formulations and using the heater, that many people see their mileage
drop in winter. During cold weather, the engine has to run even more
to maintain proper temperature.

Priuses sold in some places outside the US have an "EV" button that
forces the car to run electric only at low speed (there are conditions
that override this). Emission requirements are really the only reason
this capability is not installed on US verisons (though it can be
added). I don't know if HyHi has a similar system on overseas versions.