By any measure, the iPhone 5 had a great launch weekend. In the first three days of its availability, Apple sold 5 million units of its latest smartphone. In fact, Tim Cook asserted that Apple has sold out of the initial production run of the iPhone 5. This was a substantial improvement over last year’s already record-breaking launch of the iPhone 4S, which saw 4 million sales in the same period.

However, Wall Street analysts note that this figure falls below expectations, and that there could be up to a million more iPhone 5s on the way to their pre-order buyers.

The shortfall was chalked up to Apple’s policy of getting a customer’s signature of acceptance before counting a sale. With plenty of pre-order customers still waiting for their phones, the actual number of devices already paid for could be somewhat higher.

However, a more fundamental issue should be the apparent supply constraints for the new smartphone. Apple may have held back some stock to ensure that they’d have some iPhone 5s to offer in the second wave of countries.

The supply issues could last for quite a while, going by analyst estimates. Piper Jaffray’s Gene Munster estimated that the supply/demand balance could be met by the end of the year. While demand in the first-release markets could ease once pre-orders get to consumers’ doorsteps and stores have more units in stock, the staggered release cadence could mean that the overall constraints would just be transferred to other markets.