Could be they're told "no one needs X amount of ammo."
Could be they think the spring in the magazine will be affected. On the sub, we only had 5 rounds in each mag (1911) to "protect the spring".
Mostly the problem was, our mags were decades old, wore out, and sub supply guys don't like to "waste" $$$ on small arms.
Maybe they recently shot some folks and just haven't had time to reload