SMA stands for Simple Moving Average.
The calculation of this may be different to what you expect and is as follows.
If you view the NPS history by month, the SMA is calculated for a rolling 3 month period.
The formula is:
SMA = Sum of Promoters (for this month + previous 2 month) - sum of Detractors (for this month + previous 2 month) / Total responses (for this month + previous 2 month)
So in essence it is the NPS for all scores in the prior 3 periods.
Some people would term this a 3 period rolling score rather than a simple moving average.