Maybe you misunderstand.
The fit issues with 7.62x39 mags can usually be fixed with a file. In fact it's often expected to have to do so due to in variation in tolerances. AK 5.56 mags are often a proprietary design to the manufacturer.
Maybe we are talking past each other or different experiences. I have two 5.56 AK's whose mags don't quite fit each other. I never had any problems with 7.62 AK's until K-USA, but they fixed that issue. I have had more issues with 5.45 mags, but that was with side to side fit, not front to back.
My K-USA 7.62 AK had to be sent back because only the mag that came with it would fit (none of my combloc mags would work). Couldn't get them to lock in. I think they readjusted the placement of the mag catch. Seems they had one type mag they function tested with. They said they fixed that oversight in their QA/QC. I wasn't the first to call. Had it back in a week or so.