VTune sampling uses statistical approach for measurement, which implies some variations in results. With a small program it might be that the system itself is affecting the consistency (I/O, paging, services running, high priority processes). I highly recommend you to check your small program with a high resolution counter. You could see the inconsistency of the results easily.
If I understood correctly, you just extended the sampling collection time up to 10s, but still profiling a small function, right? If yes, this is not correct. You'd better to extend the run-time of your function up to 10s or more (make a loop with function calls) and collect samples for the wholeloop run.
You can also isolate the loop/function measurement by using sampling collection API - you envelope the piece of code with VTune API calls and collect sampling data for this code region only for more consistency.