General questions about graduate studies admissions

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I'm getting my bachelor from a university in Greece.Our grading system is out of 10.So first of all,why is it recommended not to convert your grade at a 4 system?I mean is there any actual difference between 8/10 and 3.2/4.0 as eligibility is concerned?Also do they take in consideration the average gpa's from each ones university?I mean the average gpa at my uni is around 6.5 and grades above 8.5/10 are extremely rare.So do u think i have any chance getting accepted in any of the UC's with an 8/10 given that i'll score well on the GRE?