On a purely technical note, there's two types of receiver extensions (buffer tubes) that seem to be used on .308 pattern rifles. A large variety of factory .308 based rifles like the STAG 10 utilize a standard, carbine length receiver extension - like you see on pretty much all AR15s now. However, when using a true Mil-Spec carbine receiver extension on a .308 based rifle, you have a different overall length that the bolt and carrier assembly need to travel rearward, before bottoming out the buffer and returning into battery under the pressure of the action spring. Because they have a longer overall travel, they require a shortened buffer in order to fit within the same dimensional envelope of the carbine length receiver extension.
A few other manufacturers, most notably Knights Armament and LMT utilize a longer receiver extension - by all means they appear to be identical to a standard carbine receiver extension other than length. (a la AR15 and so on). The longer receiver extension allows the use of off-the-shelf standard AR15 type buffers.
Basically, most manufacturers have chosen to shorten the buffer and use a standard carbine receiver extension, and a few others did the opposite whereby they can use a standard carbine buffer, with a slightly longer receiver extension.
