On older magazines (and cheap ones
), the magazine lips aren't hardened and will spread out over time. It was a common problem when the P08 was still in use in the 1930s as well, one of the reasons why the switch to the sturdier Haenel-Schmeisser magazines was made.
When developing the postwar Parabellum, Mauser found that with non-hardened magazine bodies and lips, the lips and even the body itself would deform in as little as a few hundred shots. Mauser's supplier switched to a thicker steel and hardened lips to cure (or at least reduce) the problem.