Random Tech Questions

[BipolarMike]

So I was messing around with QR codes, and found that in any QR Code made from a shortened Goo.gl link (and I presume any text the same length as a shortened goo.gl link) has an area above the lower left square that can be modified or even completely blanked out! I used this finding to make this modded QR Code, with a small version of my logo embedded within it, linking to my old crappy website:



The area with the [+-] can be completely blanked out, and the code still works fine!

So anyway, my random questions are as follows: Why does this work? Do all QR Codes have a "dead space" such as this? How else can a QR Code be modified?

Edit: I found that apparently American Eagle uses this technique in their QR Code as well:



What other purposes can you think of, or you have seen, that use this modification?

Edit edit: Well it looks like everyone seems to know about this but me... https://www.google.com/search?q=qr+c...w=1366&bih=643

Last edit: Website dedicated to the creation of QR Code logo-embedding: http://www.qrhacker.com/