One doesn't have anything to do with the other. The mags are there, many millions of them.
See, the problem you're describing is all about budgets and politics. Those reserve units likely didn't budget for new mags for years, or budgeted for mags and spent the money elsewhere. Now that they are deploying, they are getting an infusion of money and are trying to buy stuff all at once, when demand is high.
Other units took their budget money and bought thousands of mags, and those mags are sitting in storage, unused.
So, why doesn't a unit with lots of mags give (or sell) them to a unit that needs them? Simple: because government doesn't run that way. Any government department, agency, or unit's job (in its mind) is to increase its budget any way possible. In order to do this, they have to show that the money was SPENT on acquiring items for the unit/department. So, it will always try to buy something new, especially when money is being given pretty freely, than to recycle, reuse, or borrow something.
M4Guru noted that his unit's priority was to airlift a TV and X-Box rather than spare mags. I assure you, this is exactly the type of thing that goes on in most units. The military is not run like a business, which must be efficient in order to turn a profit. Government agencies NEVER turn a profit, and in fact, are designed NOT to.
When a unit REALLY needs mags, they will always find a way to get them. Everything else is just politics as usual.
-Troy