What is the real meaning of the "WiFi strength" indictor?
it is just a bug of the icon at manu bar, if logging out and back is not working or even rebooting your mac, try this:


Click the icon- turn wi-fi off- then turn it back on again
Do a reboot
3 Reset the router (unplug it from the mains and leave the capacitors to drain for half an hour or so)
Do an SMC reset which appertains to 'unusual system behaviour' like yours Intel-based Macs: Resetting the System Management Controller (SMC)
There is some advice on plists here https://discussions.apple.com/thread/4149916 but it concerns an aberration post-Mavericks install with regards to the presence of an exclamation mark on the icon. Apple hid the library a couple of upgrades ago so if you want to go this route eventually then hold the alt key down whilst clicking Go on the Finder menu bar.


source: Mac Forums