Hello
my OS is ubuntu 11.10 (64 bit)
My laptop model: Lenovo ideapad z470
I use my laptop wirelessly to the Internet. But the internet speed on my laptop is constantly fluctuating, sometimes normal and sometimes the speed goes down immediately. However, they do not have this problem in Windows 7

There are many variables here, almost all of them external to your devices. Remember that ultimately you are talking about radio signals and they are subject to interference from the environment.

Also, unless you are using the same software for metering signal strength in both enviroments, different outputs could be the result of different metering and reporting methodologies, rather than actual physical differences.

Are you noticing any effect on functionality and usage or just differences in metering reports? If it's the latter and if your connection is stable, for all practical purposes, it's not something to worry about and likely not something for which you will find a satisfactory explanation.

I have a for instance: my girlfriend and I have side-by-side wireless laptops, hers running XP and mine running Slack 13.37. Windows reports that her signal varies (she says, "I only have xxx mps" or "I have yyy mps") My wicd reports a consistently strong signal.

Yet, both machines perform equally well (at least as far as connectivity is concerned--after all, she is using Windows, and no I can't convert her--her work is an Outlook shop).