Maximum XFR speeds depend on temperature. If your temperatures are low enough, you have more chance of achieving the highest XFR speed for your processor.
3.9Ghz can be achieved on 1-2 CPU threads only and so you will only achieve this for short periods under light load when running certain apps that only use 1-2 threads. When you play games, you will see 3.7Ghz max.
If you download Cinebench and run the single core test, you may see a boost of up to 3.9Ghz on 1-2 threads.