Item weight is essential in the computation of real-time rate shipping cost. In the event that no item weight is set in the item records, NetSuite considers next in hierarchy what is set in Setup> Accounting> Shipping> Default Item Weight in Lbs.
However, right now it is not possible to set a decimal default item weight in Setup> Accounting> Shipping. Decimal numbers entered in the Default Item Weight in Lbs field will always be rounded to the nearest whole number. The ability to set decimal in the mentioned field is covered by Enhancement 89176 - Setup > Accounting > Set Up Accounting > Ordering/Shipping > Allow users to enter decimals in the Default Item Weight in Lbs field.
As an alternate solution, setup the decimal default item weight in the item record.