Despite being similar in appearance, the SB Tactical version has slightly different dimensions that prevent them from being compatible.
I have both the SB PDW and Maxim Defense setups... and everything is interchangeable. There is a length difference is between carbine and PDW length buffer and buffer tube. If you have a carbine length tube and a PDW stock/brace you wont be able to close it up to the shortest setting as it will hit the back of the buffer tube. Other than that, there are no issues. SB and Maxim are very cryptic about it, but it is all essentially the same.
Because it's not an SB product!