That is caused by Excel Floating Point arithmetic which may cause inaccurate results... see https://support.microsoft.com/en-us/kb/78113
Try Rounding the result of the inner function to one decimal to match your table entries)
=ROUNDDOWN(ROUND((SUM($H19:AK19)+$E19-SUM($H20:AJ20)),1)/$B20,0)*$B20