meter systems vs. site readings

[corneo]

I can't figure this out and maybe someone can help me understand it.
On my site I use Google Analytics and quantcast for readings. Neither system gives the same report. I mean one system says one thing and the other counters it. My server meter has totally different numbers. After talking to my server people they tell me go with the server reading, it's the most accurate. Which I will say makes sense.

Where I'm confused is: Why are these number so different?

I use google for pages viewed (does a good job)
I use quantcast for demographics.