compile the same project several times get several different timing results, why?

If I remove systemID component, and recompile with regenerating sopc system, there would be no variations for timing result any more.

Also I have tired not generate sopc system every time I recompile, the variations are gone too.

So I'm sure it is the matter of system ID component now.

Also I found that quartus 64 and quartus 32 give different synthesis result, and therefore timing. That's why I have different results from script compiling (64bit) and GUI compiling (32bit).

I agree the practical try is to find out the critical path and improve it.