How could DLLs be changeless?

[HawkOfWinter]

When rebuild a project with many DLLs, I found the DLLs will be changed even though the source code doesn't be modified.

I hope, if the source code doesn't be modified, the DLLs rebuilt will be same in binary.

[kbw]

If there could be dependencies thru header files or libraries that can cause recompilation or re-linking.