Access points list...

I am trying to get the wifi access point list nearby.I am using iwlist command but the result different each time each time I get the different list. Can anyone tell that what is the reason behind that as I think that while sitting at the same place result of iwlist should be same.
an other thing ,what does it men by quality and cell in ilwist response.
any helpful link ,reading material and guide line will be highly appreciated.
Ooo yes I am using ubuntu linux.
 
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