In the A2 Assistant I've set the battery's first level warning to 21.5v.  When flying, the A2 reports the voltage is 21.5v and the yellow light starts blinking on the A2's LED.  I immediately land the copter and check the voltage on the battery and it says 23.0v.  Why is the A2 saying it's 21.5v before landing?
Build:
* S900
* A2 - with the latest firmware: v2.7
* iOSD Mark II - with latest firmware: v3.04

In the air the battery is under load...

Technically, just as with other kinds of lithium-ion cells, the voltage of a LiPo cell depends on its chemistry and varies from about 2.7-3.0 V (discharged) to about 4.20-4.35 V (fully charged).
The battery performance may vary from different environment conditions.
For example, chemical reactions internal to the battery are driven either by voltage or temperature. The hotter the battery, the faster chemical reactions will occur. High temperatures can thus provide increased performance, but at the same time the rate of the unwanted chemical reactions will increase resulting in a corresponding loss of battery life.

The intelligent battery has some self-protection capability, only if you use it properly, it'll be OK. I think you don't have to worry too much about the battery.