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Yes, the status bar is showing in Safari but regular Finder windows behave the same.
 
I just checked on my old 13" MBA and my wife's 15" MBP and they also can't use those pixels! I never realized that. I guess it's more noticeable with having such limited resolution on the 11" MBA. I WANT those 4 pixels now!
 
... Notice the blue background showing below the Safari window.

It's the gap for the Dock.
if you switch Dock to either side - this gap fill follow ...

Apple's logic I deduce was - to give some breathing room - between the bottom end of your maximized app and the Dock - to limit false activations (both with Dock hidden or not)

In reality it fails to be enough of a buffer - so I just use darker backgrounds to distract me less.
Though there is at least one exception - Apple's own iTunes - it doesn't give a damn about restrictions imposed by OSX on any third party soft - it sits there, shows it's skirt, laughing in your face ...
also any apps you moved lower, so their bottom is even deeper - they show up too ....

And yeah, absolutely every Mac out there is the same 🙁
 
I just checked on my old 13" MBA and my wife's 15" MBP and they also can't use those pixels! I never realized that. I guess it's more noticeable with having such limited resolution on the 11" MBA. I WANT those 4 pixels now!

Switch Dock to the side - it'll give you your 4 (vertical) pix back (by taking 4 horizontally 😛😀)
 
Is it possible to completely disable the dock?

Yeah I was in that place you are in now ...
I disabled .... such thinking 😛🙄😀😎:apple:

But fret not - come next summer Lion will redeem you with some 40-60 extra pixels 😎🙄
(and a bonus 10 pix on far right 😛)
 
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