SOF Files content are Different after each time of Compilation!!!! Is It Normal??

The same initial conditions should create the same result. Initial conditions cover everything about the design, including input files, .qsf settings, etc. It even includes OS, as Linux and Windows give different results(due to different floating point calculations). One thing I've seen is designs that create a timestamp in them naturally have a different input each time. Also, there was an issue where you'd get a different result if you had a /db from the previous compile around(that should have been fixed). In that case, there were two diffferent results, i.e. if there was no /db you got result A, and if there was a /db you would get result B and could keep compiling over and over and get B.

Make two identical copies of the design and compile them. Are they really different?

(Finally, if it's a build going into production, and that doesn't happen that often, I recommend zipping it up with the /db and storing it that way. Minimally, it just makes it easier to open and go back and look at timing, reports, etc., but now you don't have to worry about regenerating it exactly either.)