Why ARM Mali Graphics (e.g t628 mp6) obtained in tests 3dmark low scores?units qualcomm obtain much better results, why?
That is far too imprecise a question. There's a description of ARM's Mali Midgard architecture at
ARM’s Mali Midgard Architecture Explored
If you have a look at it you'll see that parts can be replicated to give dramatically different performance.Other GPU's like versions of Adreno and PowerVR can also have bits replicated, and moreover they can do tasks in quite different ways which means the performance of some tasks does not map linearly to that on others when going between GPU's.