Note that the prompt wasn't fed to another LLM, but to the same one. "I wrote a program in C and gave it to GCC. Then I gave the same program to GCC again and I got a different result" would be more like it.

If there is no error on the compiler implementation and no undefined behavior the resulting program is equivalent and the few differences are mostly just implementation defined stuff which are left to the compiler to decide (but often gcc and clang do the same). The performance might differ also. It’s clearly not comparable to the many differences you can get from LLM’s output.

The output will conform to the standard & it will be semantically equivalent. You're making several category errors w/ your comparison.