You are correct that under the optional calculation for existing dwellings (220.83), the first 10 kVA of the calculated load is taken at 100%.Here's a breakdown: 220.83(A)

This section deals with dwellings with existing loads and the addition of heating and/or air-conditioning. The first 10 kVA of all other load is calculated at 100%.

220.83(B): This section deals with dwellings with existing loads and the addition of equipment other than heating and air conditioning. The first 10 kVA of all other load is calculated at 100%.So, both (A) and (B) of 220.83 involve calculating the first 10 kVA of the existing/general load at 100%. Flag for review