And you can learn more than you actually want to know about WGS84
here.
Essentially, GPS altitudes are above a shape which is close to sea level, the errors from real sea level are small. Aircraft normally use pressure altitudes, and the daily variations from Interantional Standard Atmosphere (which a perfect altimeter would indicate) are considerable.
To avoid hitting a lump of cumulo granitus GPS altitude is more useful. To avoid hitting another airborne moving object pressure altitude is what you need.