I believe the issue may be because decimal fractions cannot be accurately stored in floating point binary.
This is a good explanation of the issue from a general technical standpoint:
We're pleased to announce that we've introduced a new mode for the ROUND function in this weekend's release that addresses the problem you're seeing.
You can view the updated Anapedia content for ROUND here: https://help.anaplan.com/anapedia/Content/Calculation_Functions/All/ROUND.html
And we have some guidance notes for customers on the use of the new 'EXACT' mode of the ROUND function:
In essence - nothing in your existing models will change, unless you choose to opt-in to using the new ROUND mode by adding the new EXACT keyword.
Here's the example from the first post in this thread, showing the new behaviour: