Data in Web TimeSheet reports' custom column cannot be summed up because of null values in the rows.
If you try to sum the data of a custom column, it will not provide the summary at the bottom of the report, this is because if any one of the rows has a null (blank) value then it will not sum that row.

Resolution:
Custom column should have a IsNull(variable) formula so that it will fill all the null values with zero values.
Display whether the variable is: null (and can therefore not be used in calculations)
If the variable is null, the field displays 1 (true result)
has a value (and therefore can be used in calculations)
If the variable has a value, field displays 0 (false result)

Note:
Variable must be a name of another report column; any other value will yield a true result.
Use the below syntax to fill all the null values with zero values.
if(isnull(Column Name)=1, 0, Column Name)

Example:
To add zero values to BaseBillableAmount Column
if(isnull(BaseBillableAmount)=1, 0, BaseBillableAmount)