Accuracy?

[HansB]

Why does my Prime G2 shows different results with equal inputs? I like to understand how accurate to how many digits Prime calculates, and how many digits will be displayed. It appears I cannot trust the last digit displayed.
For instance (all textbook input, standard number format, results grouped here for better readability):

Code:

sqrt(3/4)      -> 0.866 025 403 784    (cos(30))
sqrt(3)/2      -> 0.866 025 403 785
phone calculator: 0.866 025 403 784 438 6...

sqrt(3/7)       -> 0.654 653 670 708
sqrt(3)/sqrt(7) -> 0.654 653 670 71
phone calculator:  0.654 653 670 707 977 1...

cubicroot(3/8) -> 0.721 124 785 154
cubicroot(3)/2 -> 0.721 124 785 155
phone calculator: 0.721 124 785 153 704 1...