Phone Arena has carried two experiments to find out whether general users can spot the difference between Quad HD and Full HD screen resolutions. Reuters/Albert Gea

Samsung's two Galaxy S6 models sold 6 million units in April, according to a new report from Counterpoint Research, even though the device was only available for 20 days because the phone went on sale on April 10. The two models, the mainstream Galaxy S6, and the curved Galaxy S6 Edge, come in as the third and fourth best-selling smartphones in the world for the period, behind the iPhone 6 and iPhone 6 Plus.

"The Samsung Galaxy S6 series has a chance to become the top selling smartphone overtaking Apple’s iPhone 6 series, if production issues are solved," according to the report.

Turns out, one of the key problems with the Galaxy S6 Edge in particular is supply issues, which have plagued Samsung's unique Edge screens since last year. Although Counterpoint saw additional consumer interest for the Galaxy S6 Edge, it was outsold by its vanilla cousin, mainly because of increased availability.

Still, the report indicates that the redesigned Galaxy S6 is enjoying a more successful release than its predecessor, the Galaxy S5, which had disappointing sales. Samsung redesigned the Galaxy S6 with its own chip, a non-removable battery, and significantly improved build quality. In terms of total device sales, the Galaxy S6 might not have outsold the Galaxy S5, but it had a more focused rollout, launching in 20 coutnries as opposed to 125.

"Considering that fewer countries were in the initial product release, it has had a slower roll-out and lower marketing budget than the Galaxy S5, the Galaxy S6 series can be considered to be performing better than the Galaxy S5," Counterpoint Research director said in a statement.

Still, both Galaxy S6 models fell behind the juggernaut in the smartphone space, Apple's iPhone. According to the report, both iPhone 6 models combined sold 51 percent more than when combining iPhone 5S and iPhone 5C sales from the same period last year.

Here's the full list of top smartphones sold in April from Counterpoint Research:

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One notable absence: Google and Motorola's Nexus 6. Counterpoint Research