By Sam Oliver

Published: 03:50 PM EST (12:50 PM PST)

Through the first two months of Apple's third quarter of fiscal 2011, U.S. Mac sales are up 15 percent year-over-year, but that number is expected to rise in June when Apple faces a more favorable 2010 comparison.

The latest NPD data for domestic Mac sales shows sales up 15 percent year-over-year in April and May, according to Piper Jaffray analyst Gene Munster. That number is less than the 22 percent jump from 2010 Wall Street expects to see in the June quarter.

But Munster noted that the "slow start" to the quarter is "too early to make a call," because Apple's Mac sales face a favorable comparison in the month of June. In 2010, new MacBook Pros in April led to a 38 percent year-over-year increase in sales, while a MacBook refresh in May helped push sales up 35 percent from 2009.

Munster noted that the comparisons "ease dramatically" in June, when in 2010 Apple saw sales increase by 11 percent year over year. He expects that Apple will close the gap and be much closer to Wall Street's expected 22 percent increase when June is finished.

Munster noted that in the April quarter, Apple reported Mac sales up 28 percent, very close to the U.S.-only figure of a 27 percent increase reported by NPD. And highlighting the importance of the third month in the quarter, NPD figures showed Mac sales up just 16 percent after the first two months of the April quarter.

The latest number, representing both April and May, is also a marked improvement from the April-only numbers previously reported by NPD. In that data, the tough comparison from 2010 left Mac sales up just 9 percent year-over-year.

During the current June quarter, Apple released new iMac desktops with Intel's latest Sandy Bridge quad-core chips, the new high-speed Thunderbolt port, and a FaceTime HD camera. But desktop Macs are a much smaller part of Apple's business when compared to its popular notebooks.

On that front, Apple is expected to release new MacBook Air models in the near future, also powered by Intel's Sandy Bridge processors. Last week, AppleInsider reported that Apple is set to build 380,000 units of its thin-and-light notebook this month.