why is airport bar not the same on the istat's

[Twe Foju]

so im using a adsl modem with wireless on it

but the status bar on the upper corner showing only 3 bars

where on the istat's its showing full bar, so i thought it's the signal problem

but when i check on my sis's G4 ( just beside my MBP ) it's showing full bar on both istat and on the toolbar

is something wrong with my airport? or istat are making a wrong calculation?

 

[fsckminix6]

i believe the one in OSX is throughput being speed and the one on istat is the signal strength. someone correct me if im wrong.