Did Anything Change?

I was driving a car along a 3-lane street. I wanted to change lane from the rightmost to the center. Before changing I took a look in the mirror, and noticed a blue car in the center lane behind me, far enough for me to change lane. After I had changed to the center line, I checked out the mirror again. The car behind me was exactly in the same location, in the lane on my left. For a while I felt perplexed: didn’t I just change the lane?
Soon I noticed that the driver of the car had took the left line exactly at the same time I took the center line.