It could be the heat or heatsoak. My car is a lot slower when it's hot out. Even slower when the engine is hot and you're sitting in traffic. Try it on a colder day and you could easily knock off half a second.
As for the G-Tech readings. I've got one and I don't think it's accurate or consistent. Go to the track and find out for sure. There you'll get 60', 330', 1/8th mile, 1000'(some tracks) and 1/4 mile intervals. That will show if there's a problem for sure. And to get good times, leave the hood up between runs to cool off. Experiment with longer cooling times. In the summer heat it can take over an hour for the intake to cool down enough to touch. Some ice on the intake will help too. Just make sure to not drip water on the engine or the track.