One reason is that it your ICE needs to run longer to do the initial warm up when starting up. This obviously affects short trips more than long. Also, if you need heat/defrost, the engine may need to idle at times to heat the cabin when in warmer weather it would shut off. I think this wouldn't be an issue if you are driving steadily so the engine keeps running but would be more of an issue in stop and go driving when the engine might shut down if the heat wasn't required.
As Irisius said, yes, could be the heater, particularly with short trips. I bought mine in winter, and if I had the cabin temperature set at 23 from the afternoon before, as soon as I hit the START, it would think - "hey, it's on 12 deg, and he wants it 23, so I'll have to turn the ICE on straight away to provide heat for the heater". I tried to remember to turn the temperature to MIN at the end of the previous day, and the ICE didn't always start up. But longer trips, that shouldn't be much of a problem.
Another factor not mentioned is the need to heat up the catalytic converter. The Prius has two design goals: fuel economy and low emissions. Posted via the PriusChat mobile app.
Also the tyre pressures drop when the ambient temperature drops. It's well known that one way to increase mileage is to increase tyre pressures. Pump up your tyres! Oh, and you can save a bit by turning off the A/C system and drive that way until the coolant warms up, -then- turn it on for heat. If you have a Scangauge you can watch to see when the coolant is warm enough (50C) that the engine won't be started by the demand for heat.
