Wessman,
The measured voltage drops when a load is applied to any battery.
What you measure with your voltmeter is no-load potential differential between the terminals, which is normally as high as 1.5 volts per cell.
The manufacturer has tested the servo with 4-cell and 5-cell batteries (at 1.2 volts per cell normally), measuring the time that a full deflection of the shaft takes.
Note that higher voltage means more torque and speed from the same servo, and also higher Amps consumption in the same time.
That extra flow of electrons leaving the battery is something to be careful about when you decide using 5-cell packs: they need to have higher mAh rating than similar 4-cell pack.
Even more, those specified speeds are at normal mechanical loads which the servo is designed to overcome.
For undersized servos working against huge control surfaces flying at high speeds, that deflection speed would be slower than specified, reaching zero in extreme cases.
That is the reason for which we should use loaded voltmeters to verify the state of the flying packs before each flight.
A non-load measurement can tell you that a battery is full, while in reality, that battery will not be able to stand the demand that loaded servos will create in flight.
This website explains more and better about batteries:
http://www.hangtimes.com/redsbatteryclinic.html