Perhaps this is the reason.
The non-farm optional method is available for determining net SE earnings from nonfarm business and is available when net nonfarm profit is less than $7,849 and also less than 72.189% of gross nonfarm income. in addition, SE income must have been $400 or more in 2 of the last 3 years. it can not be used for more than 5 years, whether or not consecutive.
