When Germany mobilized for war in July of 1914, they were already behind the curve in terms of materiel for the troops. It's why some Reserve units went to war with the M1888 Commission rifles or M1871/84 Mausers. We're talking millions of young German men needed everything... uniforms, weapons, equipment, and.... helmets. If you look at the list of manufacturers of the metalhelm during this time, they were a bunch of small metal shops cranking out these helmets as fast as they could make them, which was still not fast enough for the troops that needed them. That's why we see one-offs... a different style of helmet used for a regiment that would not normally use that particular style.
In modern times, I have experienced this during the war on terror years, when we didn't have enough equipment for everyone being deployed.... we often had to wait... we deployed to theater with what we had, and were retrofitted with the equipment, or we didn't get it until we got back stateside. In any case, it's what happens when the emergent need outstrips the existing supply.
Hope this helps.
Cheers,
Tim