If you thought Japan's NTT DoCoMo (NYSE:DCM) could reverse subscriber losses by finally offering Apple's (NASDAQ:AAPL) latest iPhone lineup to its more than 61 million cellular subscribers, think again.

NTT DoCoMo may still be Japan's single largest cellular service provider, but the company said Monday that it lost a net 66,800 customers in September, its largest-ever monthly decline. 

Worse yet, this occurred despite the fact that NTT DoCoMo proudly began carrying Apple's latest iPhones for the first time last month.

Here's the culprit
So what happened? According DoCoMo, we should look no further than low iPhone 5s inventories. This, in turn, caused many frustrated consumers to continue defecting to the second- and third-largest Japanese carriers in KDDI and SoftBank, both of which apparently had more than enough iPhone inventory to go around.

As a result, KDDI just reported a September subscriber increase of 232,700, bringing its total to more than 39 million customers. SoftBank, for its part, grew even faster by adding 270,700 subscribers, which brought its total customer base to more than 34 million.

That raises the question, then, of why DoCoMo didn't have sufficient iPhone inventory to meet demand. After all, Apple and DoCoMo should have been able to anticipate that pent-up demand considering that DoCoMo's enormous customer base has watched in envy since SoftBank became the first Japanese carrier to sell iPhones way back in 2008. Then, much to DoCoMo's chagrin, KDDI joined the Apple bandwagon in late 2011 after watching SoftBank steadily gain ground over the preceding three years.

Now that DoCoMo's finally on board, there also certainly shouldn't have been any technical network limitations -- after all, it uses the same W-CDMA 3G technology and 2100 MHz frequency band that SoftBank employs. In addition, note that this is also the same model Apple says supports the LTE networks of all three Japanese carriers.

In the end, it appears to be a simple issue of how, exactly, Apple decided to allocate its new iPhone stock among the three companies.

Of course, we shouldn't forget this could also be detrimental to a small extent for Apple if there unmet demand still exists, especially since the folks at Cupertino don't particularly care if subscribers switch carriers as long as they're still buying iPhones.

When the rubber hits the road, however, it's definitely a bad sign for DoCoMo when carrying the iPhone still isn't enough to keep its subscribers around.

But who knows? There's always the possibility that DoCoMo's subscriber numbers will start to tick upward again when it corrects its supply issues. But, to me, it seems an equally likely scenario that maybe, just maybe, DoCoMo's customers have simply had enough of waiting for their company to give them what they want.

That's why, until it becomes clear one way or the other, DoCoMo stock remains a bet I'm unwilling to take.

Fool contributor Steve Symington owns shares of Apple. The Motley Fool recommends and owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.