It is fine. This is something I have never understood as well. The fully charged voltage of a battery is different from the voltage on the label. The voltage on the label is the "nominal" voltage, where the battery tries to keep its voltage level at.

LiPo batteries have a fully charged voltage of 4.20V/cell (8.4V for a 2S pack).
The nominal voltage is 3.7V/cell (7.4V for a 2S).

When you put the battery under load (motor) it will drop close to the nominal voltage.

Go have fun!