Even though section scores in both GMAC's and 800Score's CATs are same for two full test; 50 quant-25 verbal for first test and 49 quant-23 verbal for the second, the total score they calculate differ from each other. For the first test Gmac calculate total 630 whereas 800Score calculate 640 and for the second test Gmac calculate total 600 whereas 800score calculate 620. As you see 800Score gives you 10-20 more score in total for the same section score than GMAC. If we assume that GMAC is more accurate, should I diminish my score around 10-20 for the rest of the test of 800Score's?

Practice tests generally differ by 10-20 points. You should not spend too much time focusing on this margin of difference because they are not exact predictors for the actual exam. Rather, you should use the practice test to identify general areas of weakness and focus on strengthening them.