This is an old problem of chart's editing in Calc. When you enter "constant 
value" for error bars (Data Series > Statistics), it is rounded to decimals. 
For instance, if you enter 0.067 you will get 0.1. This has been a subject of 
8877 issue. However, first, that issue has been posted for Linux but is actual 
for Windows too. Secondly, 8877 issue appeared in 2002, was tracked till 2005 
but still remains unsolved.

It is very important for scientific calculations and plotting to have real, non-
rounded error bar values in charts. More over, there is a big discussion now 
about adding so long-awaited possibility of entering custom error bars taken 
from spreadsheet cells. I am afraid if this feature is implemented finally, all 
values taken from cells will be rounded as well. I think this bug really needs 
to be fixed to prevent possible problems in future.