If all the dots line up and conditions are right the car can do a 4-2 downshift if need be. It's not going to happen all the time, normally drops back to 3rd for passing.

I figure you were doing slightly slower than 70 when you kicked it down, thus being able to drop 2 gears to give you the most power needed at the time. It only does that because your foot was prolly straight on the floor and car had to do something with the command you gave it.