Why does this memory address %fs:0x28 ( fs[0x28] ) have a random value?

On x86_64, segmented addressing is no longer used, but both the FS and GS registers can be used as base-pointer addresses in order to access special operating system data-structures. So what you're seeing is a value loaded at an offset from the value held in the FS register, and not bit manipulation of the contents of the FS register.

Specifically what's taking place, is that FS:0x28 on Linux is storing a special sentinel stack-guard value, and the code is performing a stack-guard check. For instance, if you look further in your code, you'll see that the value at FS:0x28 is stored on the stack, and then the contents of the stack are recalled and an XOR is performed with the original value at FS:0x28. If the two values are equal, which means that the zero-bit has been set because XOR'ing two of the same values results in a zero-value, then we jump to the test routine, otherwise we jump to a special function that indicates that the stack was somehow corrupted, and the sentinel value stored on the stack was changed.

If using GCC, this can be disabled with:

-fno-stack-protector