I would be surprised if manager ever did exact arithmetic.
It is much more likely decimal numbers are converted to a binary approximations. Then microprocessor approximation are used for addition, subtraction, multiplication and division. Multiplication results in twice as many digits, those beyond the number format are discarded. Division often result in an infinite number sequence, those beyond the number format are discarded. Subtraction can result in numbers much closer to zero with a very low relative precision. The binary answer is then converted back to a decimal approximation.
Lubos has been hiding this detail mostly for currencies by rounding the result to the nearest cent (the exception include GST on items not being the same as GST on the total, and currency conversions). Spreadsheets also hide the detail by rounding to fit the display cell size and allowing user specified display precision.
With user calculations in manager, the units are not known to Lubos so he can’t round to hide the calculation/ number format conversion approximations.
The solution is to support user specified rounding precision. So this should be an enhancement request/ idea not a bug report.
Computers can do “exact” arithmetic, however the answer display would not be suitable for the typical Manager user and require a major complexity programming enhancement for Lubos.
PS
Double precision floating point numbers have a precision of about 14 decimal digits. As this is the most common floating point format, it is probably what is used by Manger.
Lubos may be able to display number rounded to say 10 significant decimal digits and hide most floating point calculation limitations.