Description:
When you try to calculate with the rounded off numbers instead of the internal exact values doing this:
    Choose Tools - Options' - LibreOffice Calc'.
    Go to the Calculate page. Mark the Precision as shown field and exit the dialog with OK.

You find that SUM function just calculate real precision not shown one




Steps to Reproduce:
Copy this numbers on a column:
28,0555
82,4155
95,2355
22,3255
57,1655
2,3755
50,9597
91,0818
29,4760
5,9146
82,3533
63,1918
53,1243
90,6875
79,8511
47,6102
76,6637
76,5225
30,6896
55,3835
13,6464
86,4717
52,6737

show to digits precision an SUM all numbers. You will get 1273,87 but it should be 1273,88

Actual Results:  
SUM gives you 1273,87

Expected Results:
with two decimal shown precision SUM should produce 1273,88


Reproducible: Always

User Profile Reset: No

Additional Info:
I attach an ODS with example


User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0