It's not so much the variables, but that compilation must be a lossy process. There may be many ways to interpret the assembly, and the compiler might generate different asm than the one that was decompiled.
Much less lossy than you probably would think if you've never dug through what your typical compiler outputs. There usually aren't that many ways to interpret the assembly and it doesn't matter whether it generates different assembly as long as it does the same thing.
The typical code generator for a compiler uses all kinds of boilerplate for common constructs (loops, function calls, data access) and once you know about these you can usually recognize them on sight.
Optimizing compilers can make this quite a bit harder by the way, extra passes that do all kinds of reshuffling to get rid of instructions, to combine them and to move things from memory into registers.
You can usually tell compilers to output assembly code, doing that for a program that you wrote yourself is a good exercise to see how your high level code translates into lower level code. And with optimization off all you see is the code generator's output.
Yeah this is where you start to see decompilers outputting C code that just looks like assembler written out as C. There was one I used to use years ago - can't remember the name, some odd commercial thing from one of the many commercial C products that never really took off - that would do a good job *mostly* but at some point start outputting blocks of code with lots of `register` variables in, and you knew it had gone out to lunch on that bit.
