Even Non-destruction method does not damage a lot on the surface but we have to use Schmidt rebound hammer all the grid that we built it on the surface of the concrete so we will have a lot or value of Schmidt rebound hammer and we use the average value. Thus the error will have less than the direct method.
Non-destructive method have to measure all of the surface so the value we get will less of error.
Conclusion
We get the correction factor about 0.99 that mean the value that we measure firstly is more than enough. The Ultimate value of concrete is about 647.87 ksc and the cube compressive strength is about 428.28 ksc. Why the value is different because I think the cube compressive strength will not make the cube concrete damage or failure but the ultimate made.