pen drive Speed Increase when using USB 2.0 Pen drive in a USB 3.0 Port.

[artemiskiss...]

So i came across an old USB 2.0 Pendrive which had some important data on it. I was transferring it onto the Laptop HDD using TERACOPY 2. I noticed that the read speed, when the pendrive was in a USB 2.0 port, was 16 Mbps for large files (Movies >2GB). So then, for experimenting, i transferred a copy of the movies back on the pen drive and this time, stuck it into the USB 3.0 port on my laptop, and copied them back to the HDD.

Now, i am aware that the greater power delivery on a USB 3.0 Ports is due to the additional connectors present and should only work with USB 3.0 devices (and i also know about backward compatibility). HOWEVER, surprisingly, i recieved a speed of 17 Mbps this time and once or twice, it even showed 18 Mbps for a second. 

CAN ANYONE GIVE ME REASONS FOR THIS?

BTW, i tried this experiment 3 more time just for eliminating any errors that occurred the first time. It didnt touch 18 this time, but stayed on 17.