AdDuplex's data is drawn from its advertising network on May 1st (500+ apps running AdDuplex SDK version 2.0 for the May report). The data essentially represents a sampling of the active installed device base for Windows Phone, albeit one biased to those users who are more likely to install apps.
Given their debut dates in the Indian market neither the Nokia Lumia 520 or 620 appears in AdDuplex's device breakdown for India in the March report, which is shown below. This suggests that, in India, the Nokia Lumia 520, in around 25 days, and the Nokia Lumia 620, in around 55 days, exceeded the sales of any other Windows Phone device, despite these other devices being available for much longer periods (e.g. Nokia Lumia 710 has been available for more than 12 months). Together the Nokia Lumia 520 and 620 make up roughly a third of the active Windows Phone devices in the Indian market.
These numbers provide compelling evidence that, in certain markets, lower cost devices are crucial for the growth of the Windows Phone platform. It's also help explains why Nokia believes it can grow it sales in the second quarter of the year at the same rate as in the first quarter, despite the launch of products like the HTC One and Samsung Galaxy S4.