Since POI uses 'double' datatype to perform calculations the sum of the following arguments (5.6, 5.8) will be 11.399999999999999. Here is the explanation why we get such result when we sum up those arguments: https://stackoverflow.com/a/322875. Consider using BigDecimal instead of 'double'.
Excel uses doubles under the hood. POI is an attempt to approximate what Excel does. If you want exact results instead of floating point approximations, please do not use Excel or POI. What Excel does is use formats to define how the approximate values should be displayed. It's hard to imagine how they came up with this but this is what we are stuck with. Use POI's DataFormatter to apply the cell formats. Closing this issue as: Won't Fix