There has been changes in the loadings from Wolf, Tula, Barnaul etc. over the years. Different case coatings, thicker rims, slight bullet weight changes (122, 123, 124 as common FMJ). Putting current production rounds side by side, there are small variations in overall length between brands for same weight bullets. Store branded runs like Monarch, Herters, HotShot and others including the different Bear and Tiger products set their specs with the manufacturers (most are made by Barnaul) so they may have differences in powder speed, flash suppressants, and velocities as well. So in general, yes they are all similar yet there can be a lot of differences in people's experiences with the Russian 762x39 depending on when and by whom it was made. The good news is that at least for 762x39 they are close enough that most users with SKS or AK pattern guns don't notice. When you get into other calibers where guns may be more inherently accurate with closer tolerances like 223 or any number of other 30 caliber rounds then the changes from lot to lot and maker to maker may be significant.
(For whatever it's worth, I have yet to get a single dud 762x39 since I began shooting the stuff in 1990 - Chinese, Russian, east European whether surplus or commercial - I think close to 35k of the stuff. Other calibers from over there, definitely had bad lots with poor performance, ripped cases, duds, and mis-sized rounds that would not feed so although they might have the standard service round down pat they others leave much to be desired at times.)